Question 332797
John, twelve years old, is three times as old as his brother. How old will John be when he is twice as old as his brother? 
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Let John's age = J = 12
Let John's brother's age = B
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TODAY
J = 3*B is given
since J=12, then B=4
Today John's brother is 4 years old
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future:  X years away
J+X is John's age x years away
B+X is John's brother's age x years away
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How old will John be when he is twice as old as his brother? 
we want to find  J+X
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J+X=2*(B+X) we know today J=12, B=4 so substitute and solve for X

12+x=2*(4+X)
12+x=8+2x
4=x  (four years in the future the above relationship will hold true

so J+X = 12+4=16

When John is 16 he will be twice his brother's age (B+X) = 4+4=8