Question 1185848
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This one is a good exercise to solve in your head...














Current ages can be fully determined:
Mensah is 5yrs old --> Joyce is 3*5 = 15yrs old

So clearly, in 5yrs, when Mensah is 10yrs old, Joyce will be 20yrs old and twice Mensah's age.

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The algebraic setup, showing every step in detail: 
   Let  J = Joyce's age
   and  M = Mensah's age

(1)    J = 3*M = 3*5 = 15   (for current ages)

(2)    2(5 + x)  = 15 + x   (for future ages, unknown is "x" years for condition 2xM = J to be met)

    Multiply left side thru by 2:
    10 + 2x = 15 + x

    Subtract x from both sides:
      10 + x = 15

    Subtract 10 from both sides: 
       x = 5