Question 1111470
Bobby is 3years older than Chris last year the sum of their age was 39,how old each now 
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Equations:
B = C+3
B+C = 39
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Substitute for "B" and solve for "C"::
C+3 + C = 39
2C = 36
Chris = 18 yrs.
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Solve for "B"::
Bobby = C+3 = 18+3 = 21 yrs
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Cheers,
Stan H.