Age problem {ACT}

Bob's current age is 23 years and Melanie's current age is 15 years.  How many years ago was Bob's age twice Melanie's age?
A.) 16
B.)  9
C.)  8
D.)  7
E.)  5

x = number of years ago 
23 - x = Bob's age "x years ago"
15 - x = Melanie's age "x years ago"

23 - x = 2(15 - x) {how many years was Bob's age twice Melanie's age}
23 - x = 30 - 2x {used distributive property on right side}
23 + x = 30 {added 2x to both sides}
x = 7 {subtracted 23 from both sides}

D.)  7 years ago, Bob was twice as old as Melanie

To check:
7 years ago, Bob was 23 - 7 = 16
7 years ago, Melanie was 15 - 7 = 8
He was twice as old as her

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