When the average age of a couple and their son was 42 years, the son married and got a child after one year. When the child was 5 years old, the average age of the family became 36 years. What was the age of daughter-in- law at the time of their marriage ?

Sum of the ages of father, mother and son at the time of son's marriage
= (42 × 3) years
= 126 years
Sum of the present ages of father, mother and son
= (126 + 3 × 6)years
= 144 years
Sum of the present ages of father, mother, son and grandson
= (144 + 5) years
= 149 years
Sum of the present ages of father, mother, son, daughter-in-law and grandson
= (36 × 5) years
= 180 years
Daughter-in-law's present age
= (180 - 149) years
= 31 years
∴ Age of daughter-in-law at the time of marriage
= (31 - 6) years
= 25 years