A father is K years old and his son is M years younger. The sum of their ages is 53.

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A father is K years old and his son is M years younger. The sum of their ages is 53.

Father's Age = K
Son's Age = K - M

and we know K + (K - M) = 53

Combine like terms:
2K - M = 53

Add M to each side:
2K - M + M = 53 + M

Cancel the M's on the left side, we get:
2K = 53+ M

Divide each side by 2:
2K/2 = (53 + M)/2

Cancel the 2's on the left side:
K = (53 + M)/2