Question 232912
John is twice as old as his friend peter,                   J=2P
Peter is 5 years older than Alice,                          P=A+5
In 5 years John will be three times as old as Alice,      J+5=3A  


J=2P
P=A+5
J+5=3A
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J=2(A+5)
J=2A+10
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J+5=3A
2A+10+5=3A
10+5=A
15=A
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P=A+5
P=15+5
P=20

So peter is 20, lets check our work.  If peter is 20 then John must be 40.  In 5 years John will be 45, which is 3 times as old as Alice who is 15.