M = 5*D (1) (M-5)*(D-5) = 105 (2) Substitute (1) into (2): (5D-5)*(D-5) = 105 (D-1)*(D-5) = 21 D^2 -D - 5D + 5 = 21 D^2 - 6D - 16 = 0 (D+2)*(D-8) = 0. The positive root D= 8 is the only meaningful solution. Answer. Daughter is 8 years old. Father is 40 years old.