Question 1188489
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Let x be the number of years to pass.


Then your basic equation is this

    40 + x = 2*(10+x)


Simplify and find x


    40 + x = 20 + 2x

    40 - 20 = 2x - x

      20    =    x


<U>ANSWER</U>.  In 20 years.


<U>CHECK</U>.  In 20 years their ages will be  10+20 = 30 (the son)  and  40 + 20 = 60  (the father).
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Solved.