Sig Fig: Calculate (.82+.042)(4.4*10^3) - 3800

[cuj93]

Homework Statement

Calculate (.82+.042)(4.4*10^3) keeping only significant figures.

The Attempt at a Solution

I got .82+.042=.86
(.86)(4.4*10^3)=3784
So sig. fig says the answer should be 3800. Please explain what I did wrong. Thank you.

 

[BvU]

Clue is in the number of significant figures they give you in the numbers provided. And they did a good job writing e.g. 4.4*10³ instead of 44000 to make clear the number of significant digits...

And you do get the correct idea writing .86 and not .862 !

 

[cuj93]

Clue is in the number of significant figures they give you in the numbers provided. And they did a good job writing e.g. 4.4*103 instead of 44000 to make clear the number of significant digits...

And you do get the correct idea writing .86 and not .862 !

So .82+.042=.86 according to sig. figs., then (.86)(4.4*10^3) would equal 3.8*10^3 since .86 has 2 sig. figs. and 4.4*10^3 has 2 sig. figs.?My professor has a program called drillmaster and according to this program the answer is 3792

 

[BvU]

The program can't understand what is asked in the exercise. It can only process the given numbers. At that, it does a pretty bad job too: at least it should not truncate 3792.8 to 3792 but round off instead.

So .82+.042=.86 according to sig. figs., then (.86)(4.4*10^3) would equal 3.8*10^3 since .86 has 2 sig. figs. and 4.4*10^3 has 2 sig. figs.?

Almost. You don't really want to do the rounding off at every intermediate step. 82 42 and 44 are 2 siginificant figures, so from the calculated result 3792.8 you want to round off to two as well: 3.8 10^3.

Note that generally one provides an extra digit if the first is a 1. So 98 + 3.2 is not 100 but 101.
(or rather 10.1 * 10^1).

 

[cuj93]

The program is not like a calculator. It gives the question then a list of answers and you select one. I can try to post a pic.

 

[HallsofIvy]

Each of the numbers given has two significant figures so the answer must have two significant figures. Only one given answer has two significant figures- that is a: 3800. Are you saying that you checked (a) and were told it was wrong? If so, discuss this with your teacher.

 

[cuj93]

That is what I am saying. The program says the correct answer is c.

 

[Borek]

So the program is wrong. Note, that it almost for sure uses predefined questions (that is, it doesn't calculate the answer, it has it on file; whoever entered the answer into the file, made a mistake).

 

[cuj93]

Thank you everyone.