[jaccia]

I need assistance with determining the significant figure in my answer.

My problem stated: 10^-15 / 4.2 *10^-7 = I got .000000002

Then i need to state the answer in 3 significant figures.
Is the answer 2.00 or .000 ?

Then i have a question regarding this problem:
4.3 x 10^-6 is between what two numbers below?
1x10^-7 and 1x10^-8
1x10^-6 and 1x10^-7
1x10^-5 and 1x10^-6
1x10^-4 and 1x10^-5


Lastly, which of the following is equal to this problem   5x10^10

500x10^8
50x10^9
50x10^11
0.5x10^11

[sjb]

Look into the difference between significant figures and decimal places.

[jaccia]

What does that mean?

[sjb]

See, for instance http://en.wikipedia.org/w/index.php?title=Significant_figures&oldid=366389548#Rounding