# This might be serious

And while I think you should definitely try to improve your on-the-fly number crunching ability, remember that being a good mathematician does not mean being a human calculator.

tiny-tim
Homework Helper
It happened again today. A friend of mine and I were talking about something and he was trying to numerically give me an idea. so as he was making his point he was like what's 15 x 3.

But I was always pulled out the class for ESL during the time she was teaching the math so I missed out on it and never learned it. I'm not sure about the places with the digits of numbers. What's tens, ones, thousands, ten-thousands. It wasn't until 9th grade that I parted ways with ESL.

So now I have to re-learn all the math I didn't learn in grades 3-5
If you ask me what's 15 x 3, I immediately know that it's 45, and I expect your friend does too.

There are quick ways to do mental arithmetic, but it shouldn't be necessary for simple questions like that.

From your history of being pulled out of maths class, it seems unlikely you have discalculia … you simply need to practise. Try the 15 25 35 and 45 times tables for a start (then 13 17 and 19, then …).

(and yes I know it's a drop in the ocean … but you have to start somewhere )

Yeah. Thank you ALL for the advice. I'll look online for some basic math stuff. I like the idea of asking myself questions as I walk outside / through the supermarket and stuff.

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In addition to the suggestions given, I would try to find yourself a "math tricks" kind of book. I found a good one (although I'm on campus right now and the book is home, so I don't remember what it is called) that shows you ways to quickly multiply, divide, or estimate answers.

On top of that, what people said: practice. I had a similar problem truthfully, not as bad but simple things would catch me offguard. But I did exactly what others are saying, just calculating things in my head everyday. Not to the point where it's obnoxious, but when an appropriate situation arises.

Simple things like, I'm going 85mph, the exit is in 5 miles.. how long will it take me to get there..

just to get your mind going, even just estimating, getting used to how numbers act with others and such.

and also, a helpful (albeit simple) hint for multiplication and the like.. is to split up the number.

15 x 3 is 10 x 3 + 5 x 3... to this day, that's how I see it in my head, and I can figure it out quickly.

learn to split things up when adding too, 26 + 17, to me is immediately (20+10)+(6+7)... in my head, I'm thinking rapidly that 6 needs 4 more to be 10, and then what will be left of 7?

Do these things all the time, in different situations, and you'll grow to be more and more comfortable with it.

Just to add as well, the most interesting thing is that once you get these simple computations down, and are used to doing math in your head occasionally every day.. you'll find yourself applying higher math and math applications to everyday life.. which is VERY cool...

I know math was interesting to me, but then I began to take calculus based physics, and my mind went CRAZY analyzing and computing things in everyday life. Just estimates, and rough ones at that, but seeing the connection between what I was learning in the classroom and real/everyday life was mind blowing.

Yeah. Thank you ALL for the advice. I'll look online for some basic math stuff. I like the idea of asking myself questions as I walk outside / through the supermarket and stuff.

I can't advocate this enough. You might find it slow going, and when you remind yourself to check with a calculator the answer of some question you had at the supermarket, you might get it wrong. But the main thing is to practice. Also try to visualize/sense. Someone above mentioned that given 15*3 he instantly thinks 45. Given that we work in base 10 and that almost everyone in the developed world is familiar with analog clocks, that's not surprising. An everyday calculation like (3/4)*60 is great practice.

cobalt124
Gold Member
Agreed. And it can be fun too!

In addition to the suggestions given, I would try to find yourself a "math tricks" kind of book. I found a good one (although I'm on campus right now and the book is home, so I don't remember what it is called) that shows you ways to quickly multiply, divide, or estimate answers.

On top of that, what people said: practice. I had a similar problem truthfully, not as bad but simple things would catch me offguard. But I did exactly what others are saying, just calculating things in my head everyday. Not to the point where it's obnoxious, but when an appropriate situation arises.

Simple things like, I'm going 85mph, the exit is in 5 miles.. how long will it take me to get there..

just to get your mind going, even just estimating, getting used to how numbers act with others and such.

and also, a helpful (albeit simple) hint for multiplication and the like.. is to split up the number.

15 x 3 is 10 x 3 + 5 x 3... to this day, that's how I see it in my head, and I can figure it out quickly.

learn to split things up when adding too, 26 + 17, to me is immediately (20+10)+(6+7)... in my head, I'm thinking rapidly that 6 needs 4 more to be 10, and then what will be left of 7?

Do these things all the time, in different situations, and you'll grow to be more and more comfortable with it.

I did the 85mph divided by 5 in my head. At first I got a bit discouraged when I tried to go 12 x 5 = 60 so I can keep going 65, 70, 75, 80 'til I reach 85 but I felt that would be too long and didn't see myself doing that unless I used my fingers as an aid.

So I divided it in my head by visualizing,:

I went 5/85

5x1= 5 (8-5 = 3) bring down the 5 and becomes 35. So 7x5= 35 and I'm left with 17 at the top. Then I checked it in the calculator and it was correct. Took me a bit because I wanted to give up at first when I thought about the idea of counting 65, 70, 75. the problem though, in a real life situation I feel like the clock is ticking and the longer I take I'll get this "are you stupid?" look so I feel pressured and I can't take my time to do it. In real life if I couldn't fake my way out, I would have said the answer is somewhere around 15-18 just as a rough estimate / guess because I know taht 12x5 = 60 so going up by 5's the answer will lie in the range of 15-18

Really great tip with the going by 10s suggestion. It's really helpful. For the 26 + 17 using your 10s suggestion I would quickly have went 20+10 = 30 and 7+6 = 13 because 7+7 = 14 so 7+6=13. it's easier adding anything to 10 because you don't have to worry about the zero so 30+13 = 43 Thanks once again.

Now one last question with multiplication / dividing.
See how 4x5 = 20? So 400 x 500 =??? 20xxx but how do I determine the amount of zero's after the 20? what's the trick?

jhae2.718
Gold Member
400 * 500 = 4 * 5 + 0000 = 20 + 0000 = 200000
I'm abusing notation, but that should get the concept across.

More correctly, 400 * 500 = 4*10^2 * 5*10^2 = 4*5*(10^2)^2 = 20*10^4 = 2*10^5 = 200000

Or, 4x500=2000, so 400x500=100x(4x500)=200 000.

I used to be quite good in mental calculations, but I forgot most of it. Recently, I wanted to improve my mental skills, and I've found the perfect way to do it.

Basically, it is calculating prime factorizations of numbers which have at most 3 digits. You'll find these numbers anywhere: when looking at the clock and seeing that it is 3:30 gives you the number 330, the number of the train you happen to sit in, the channel on the TV that you're watching,...
So I made a commitment to myself, every time I saw a three digit number (and I had the time to do it), I would calculate it's prime factorization. It's great: you'll be doing a lot of divisions and multiplications for it!

jhae2.718
Gold Member
Factoring the time is fun.

Random http://en.wikipedia.org/wiki/Fermi_problem" [Broken] are fun too.

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tiny-tim
Homework Helper
I did the 85mph divided by 5 in my head.
I know 5 is 10/2,

so I do it by dividing by 10 and then multiplying by 2 (which is much easier! ) …

85 x 5

= 8-and-a-half x 2 = 16 + 1 = 17
Now one last question with multiplication / dividing.
See how 4x5 = 20? So 400 x 500 =??? 20xxx but how do I determine the amount of zero's after the 20? what's the trick?
It's the total number of 0s (except of course you have to be careful because the 20 has an extra 0) …

frankly, I think most people write it down and count very carefully, because it's so easy to make a mistake.

Lichdar
Practice, practice, practice.

Practicing in different places might also help, but always keep practicing. Consider it part of your entertainment.

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Long story short, I'm a freshman in college but I struggle with the really BASIC BASIC stuff in math. The other day, I nearly embarrassed myself in-front of a few students. The teacher asked me what's some 3 digit number divided by another 3 digit number and it was one of those "you don't NEED a calculator for" and I didn't know it. I was sweating buckets and the place was dead silent with everyone staring. I tried to play it off as if I didn't understand what she was saying until about 5 minutes later that I finally got the answer. The point is, I struggle with the basic stuff in math. With my calculator I can just punch stuff in on exams and I'm good to go. (that's how I got this far). But the simple stuff you don't need calculators for, I struggle with. I've done well on math before and when I use my calculator I can get things done. But without a calculator, I don't know the basics.

Ex, if someone asks what's 7x8 and I don't have a calculator I wont be able to tell them. And I'm afraid that might embarrass me in public one day. Also I don't know how to add / subtract 3 digits without a calculator. I can't help my little sister with her 5th grade homework unless I jump on Google or use a calculator. I can't quickly tell time on an analog clock and when I look at a digital one, ex supposed the time is 1:16 and you want to know how many more minutes 'til 2:00 I can't tell you it quickly. I'll have to do it in my head for a bit. When I buy something, I can't do the math in my head so I never double check if I get the correct change or not. I always take it as is. When I do try to do the math, I just focus on the dollars and ignore the change.

I'm I suffering from something? This has really been bothering me a lot these few days. It has completely killed my self confidence and motivation.
My guess: It's either one of two things...

1. Laziness
2. Innumeracy

Regarding Innumeracy: A very tiny percentage of the population (less than 5%, I think I've read) have practically no intuition for numbers or math. If you can imagine the worst possible case of dyslexia, but with numbers instead of words...that's the kind of innumeracy I'm talking about.

That said, most people who are poor at math are just lazy. However, there is that tiny percentage of people for whom no amount of studying will ever lead to mathematical competence at any level.

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Re: Innumeracy. There's a kind of innumeracy related to spatial manipulation, too. Like, you could ask someone how high the top of a door jam is, and he/she wouldn't be able to make even a decent guess. The person might be 6 feet tall and not have to duck his head as he goes through the door, yet he might guess the door jam is 5 feet tall...

diazona
Homework Helper
Yeah. Thank you ALL for the advice. I'll look online for some basic math stuff. I like the idea of asking myself questions as I walk outside / through the supermarket and stuff.
On that note, comparison shopping is a pretty good application. Calculating the price per unit quantity of food (e.g. price per quart for beverages, price per pound or per ounce for other things, etc.) can potentially save you money and it's also good practice at estimating quotients.

Just do it. Next time you have a homework assignment in anything, put your calculator away and do it all by hand. Do it enough and it will become second nature. Its very typical that in high level classes you are not allowed a calculator for exams quizzes so a little practice now will have big rewards later.