The average age of mother, father and son was 42 years at the time of the marriage of the son. After 1 year an infant was born and after 6 years of marriage the average age of the family becomes 36 years. Find the age of the bride at the time of the marriage.

25 years

Average of ( Husband + Wife + Son ) = 42 years

Total age of ( Husband + Wife + Son ) = 42 × 3 = 126 years

Total age of (Husband + Wife + Son) + Daughter-in-law+ Child = 36 × 5 = 180
⇒ (126 + 6 × 3) + Daughter-in-law + Child = 180

⇒ (126 + 18) +Daughter-in-law + Child = 180

⇒ 144 + Daughter-in-law + Child = 180

⇒ Daughter-in-law + Child = 180 – 144 = 36

∴ Age of daughter-in-law at the time of marriage  = 36 – (6 + 5) = 25 years