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John is 33 years old. His three sons are 5, 6 and 10 years old. In how many years will the three sons together be as old as their father? 
Overseas Competition->Knowledge Point->Word Problem Modules->Age Problems
Let x be the number of years needed for the ages of the three sons sum up to be equal to their dad\textquotesingle s age.  Their dad\textquotesingle s age is then 33+x, while the sum of the sons\textquotesingle{} age is  (5+x) x (6+x)+(10+x)=21+3x.  We have the following equation:  21+3x = 33+x  2x = 12  x = 6 