Question 1113756
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"If 5 year from now, John will be 16 years old", it means that John is 11 years now.


Since "John is 10 years older than David", it implies that David is 1 year old now.


Since "David and Alex have a total of 25 years", it implies that Alex is 24 years old.
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3 lines mental solution.



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I am sure that this problem is EITHER for young students  (2nd - 3rd grades) who still know nothing about systems of linear equations, 

OR for quite old &nbsp;(70 years and older) &nbsp;who use it as the therapy and logical exercises for their minds.