significant digits in math vs. science.......is there a difference?

[8filltheheart]

My kids know how to use sig figs appropriately. In our math answer key, however, they do not follow the rules consistently.. For example, (7 x 10^5)/(2 x10^-3). The kids' answer is 4 x 10^8 vs. the answer key is 3.5 x 10^8.

 

Is there a general rule about sig figs in math?

[Twigs]

I don't have any references at hand, so I will answer from my memory. My background: I have degrees in Medical laboratory Technolgy (my current occupation) and Physics (most of which I have forgotten). I've also tutuored chemistry and biology.

 

Significant figures apply when you are dealing with measurements.

For example, if you use a ruler to measure a rectangle's area and measure

Length = 1.2 inches and Width = 2.3 inches, then

area = L x W = 1.2 in x 2.3 in = 2.76 sq in.

Using significant figures (in this case, 2 sig. fig.), round the answer to 2.8 sq in.

 

When you have exact numbers (such as when counting or when doing math problems with just numbers and no measurements), significant figures don't apply.

For example, is there are 6 boys and 8 girls, the total is 14 children, with not rounding for significant figures.

 

Hope this helps.

[Twigs]

Here are some websites that explain more about exact numbers and significant figures:

 

http://student.ccbcmd.edu/~cyau1/WhatExactNumbers.htm

http://mathforum.org/library/drmath/view/68392.html

http://www.chemtutor.com/numbr.htm

[8filltheheart]

Thank you for responding. Your explanation makes sense. I should have noted in my OP, however, that this is a section on computing sig figs. The instruction in this section is rather vague and not what I would describe as accurate b/c it never even addresses the issue of 0 as a non-sig digit unless between 2 non-zero digits.

 

My kids were complaining a lot b/c it seemed like random variations and not according to the rules that they have learned about sig figs. There were questions that include measurements, etc and yes, they were correct in digits. Perhaps just like the zero discussion, the measurement discussion was simply absent.

 

Thanks again.