math
posted by susan .
aaron is 13 years older than shelby. in nine years aaron will be twice as old as shelby. how old is shelby?

A = Aarons current age
S = Shelbys current age
A = S + 13
In nine years aaron will be twice as old as shelby:
( A + 9 ) / ( S + 9 ) = 2
( S + 13 + 9 ) / ( S + 9 ) = 2
( S + 22 ) / ( S + 9 ) = 2 Multiply both sides by ( S + 9 )
S + 22 = 2 ( S + 9 )
S + 22 = 2 S + 18
22  18 = 2 S  S
4 = S
S = 4
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