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Let x = son's present age
3x+3 = father's present age
x+3 = son's age in 3 years
3x+3 + 3 = father's age in 3 years
"Father's age will be ten more than twice the son's age"
3x+3 + 3 = 2 (x+3) + 10
3x + 6 = 2x + 6 + 10
x = 10 = son's age now
3x+3 = 33 = father's age now
Check: In 3 years, the son will be 13 and the father will be 36, which is indeed 10 more than twice the son's age.
R^2 from SCC