Question 895240
Let Bob's age (in years) now = B
Let his Sister's age (in years) now = S
Let his Brother's age (in years) now = R

Problem 1: In three years, Bob will be five years older than his sister will be.  How old is his sister?
B + 3 = 5*(S + 3) = 5S + 15
5S = B - 12
S = (B - 12)/5
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Problem 2: In three years, Bob will be twice as old as his brother will be.  How old is his brother?
B + 3 = 2*(R + 3) = 2R + 6
2R = B - 3
R = (B - 3)/2