Question 835961
Jill is 3 times as old as her cousin Dennis. In 4 years, she will be twice as old as Dennis.
Equations:
j = 3d
j+4 = 2(d+4)
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Substitute for "j" and solve for "d"::
3d+4 = 2d+8
d = 4 (Dennis's age now)
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j = 3d = 12 years (Jill's age now)
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What will the sum of their ages be in 4 years?
Ans: (4+4)+(12+4) = 24 
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Cheers,
Stan H.