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Hello josemnz84 - I was looking at the alternate ways to solve this problem and I don't quite understand what you did here. Can you please explain? Specifically how did you get 10^{-14k}. I feel this is incorrect, but please correct me if my calculations are off.

If you want to cross multiply, the problem would be done this way:

\frac{15*10^{-4 + m}}{3*10^{-2+k}} = 5*10^7

15*10^{-4 + m}=15*10^{5 + k} - when you multiple powers of 10, you multiply the whole numbers and add the powers of 10 (you seemed to have multiplied the exponents rather than adding to get 10^-14 in your answer). See this: http://www.dummies.com/how-to/content/m ... ation.html

After cancelling out the "15 x 10^" (essentially 10^1) from both sides, you are left with: