Let the father's age at death be F and the son's age be S. F+S = 120. Let the son be born in year Y. The father was born in 1955-F. The son was born in 1955-S. Father's birth year must be before son's. This problem is ambiguous. Assuming it means the sum of their ages in 1955 was 120. Let father's age be x, son's age is 120-x. Son was born in 1955-(120-x) = x-65. This doesn't give a unique answer. The intended logic might be flawed, but option 1835 implies the son was 120 in 1955, which is impossible.