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