# B Forgotten correct method of basic arithmetic (subtraction)

1. Sep 19, 2016

### Logical Dog

What happens when you have to subtract:

30
-9

Basically 30 - 9 but the 3 must be made 13, so what happens to the zero? I know if it was any other number we would reduce its value by one.

Also, if possible, can anyone please show me how 30 in base 5 is 110?

I understand that its 1*5*5 + 1*5 + 0*1. But can anyone divide and show me how they arrive to it? I need to do the exercise in the book and I had the same situation as above! Took it to a friend in uni and he couldnt do it either.

2. Sep 19, 2016

### Staff: Mentor

You could look at it as an expression:

30 = 30

30 - 9 = 20 + 10 - 9

30 -9 = 20 + 1 = 21

Here's a reference to look at for base conversion first:

Last edited: Sep 19, 2016
3. Sep 19, 2016

### Staff: Mentor

What do you mean? I don't see a $13$.
How do you write $30$ in the base of $10$? It's the same procedure.
How often is $1000$ in included in $30$? Say $r$.
How often is $100$ in included in $30-1000r$? Say $s$.
How often is $10$ in included in $30-1000r-100s$? Say $t$.
How often is $1$ in included in $30-1000r-100s-10t$? Say $u$.
So $30 = 1000r + 100s + 10t + 1u$.

4. Sep 19, 2016

### Staff: Mentor

How not to do the math:

5. Sep 19, 2016

### Staff: Mentor

There's no 13 in the problem or the work.

To subtract 9 from 0, you need to "borrow" 10 from the 30, so that you have in effect 20 + 10 - 9, or 21.

6. Sep 20, 2016

### Logical Dog

I got it. Me and a couple of freinds worked it out, you have to keep dividing until the remainder is less than the base!

Sorry!

7. Sep 20, 2016

### symbolipoint

No dividing necessary and none indicated for the example. Mark44's post #5 should be clear.

8. Sep 20, 2016

### Logical Dog

Honestly I am not sure what I was thinking when I asked this, very sorry. :(

9. Sep 21, 2016

### Fervent Freyja

That's alright, everyone slips up every now-and-then, especially when stressed! I'm notorious for overthinking and overlooking things. When you get older it will worsen. I found my cell phone in the refrigerator the other day. I know I had to have done it because things have ended up in there in the past. You are just fine!

10. Sep 21, 2016

### OCR

Best definition of reassurance I've ever seen...[COLOR=#black].[/COLOR]

11. Sep 21, 2016

### symbolipoint

One gets comfortable with the simpler things as one gets older and more experienced and reexamines them, even the "easy" things. One recognizes these ideas about Subtraction and borrowing, whether whole numbers, mixed numbers (fractions) , decimal numbers, or bags of objects. The miner mental slips should be only temporary, or occasional.