Question 1062212
Let M = Mary's age
Let G = Gabe's age

M = G + 18   (1)
M-15 = 4(G-15)  (2)

Subs G+18 for M from (1) into (2):
(G+18) - 15 = 4(G-15)
G + 3 = 4G - 60
    -3G = -63
        G = -63/-3 = 63/3 = 21
M=G+18 = 21+18 = 39
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Ans:
Gabe is 21,  Mary is 39
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Check:
    (39-15) = 24
    4*(21-15) = 4*6 = 24  (ok)