While I have understood the solution given in OG, I feel there can be a simpler solution 
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Here's one approach:if (0.0015 x 10^m) / (0.03 x 10^k) = 5 x 10^7, then m - k = ?
a. 9
b. 8
c. 7
d. 6
e. 5
First rewrite the equation as:
(0.0015/0.03) x (10^m)/(10^k) = 5 x 10^7
Simplify to get: (0.0015/0.03) x 10^(m-k) = 5 x 10^7
Aside: A quick way to simplify 0.0015/0.03 is to multiply top and bottom by 10,000 to get 15/300.
Simplify to get 1/20
So, we now have (1/20) x 10^(m-k) = 5 x 10^7
Isolate 10^(m-k) by multiplying both sides by 20 to get:
10^(m-k) = (20)(5) (10^7)
Simplify: 10^(m-k) = (100)(10^7)
Simplify: 10^(m-k) = (10^2)(10^7)
Simplify: 10^(m-k) = 10^9
So, m-k must equal 9
Answer: A
Cheers,
Brent
