A father who is 42 years old has a son and a daughter. The daughter is three times as old as the son. In 10 years, the sum of all their ages will be 100 years. How old are the two siblings at present?

Answer 1

D = 3S

(D+10)+(S+10) + (42+10) = 100

Substitute 3S for D in the second equation and solve for S. Insert that value into the first equation to solve for D. Check by putting both values into the second equation.