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>>...a man is four years older than his wife...<<
M = W + 4
>>...a man is...5 times as old as his son...<<
M = 5S
>>...when the son was born...<<
That was S years ago when
the man's age was M-S and the wife's age was W-S
the age of the wife was six-seventh of her husband's age.
Therefore W-S =
(M-S)
So we have the system of equations:
M = W + 4
M = 5S
W-S =
(M-S)
Simplifying the third equation
7(W - S) = 6(M - S)
7W - 7S = 6M - 6S
7W - S - 6M = 0
M = W + 4
M = 5S
7W - S - 6M = 0
Solving that system gives
M = 35, W = 31, S = 7
Edwin