The average age of a husband and wife and their son was 40 years, the son got married and a child was born just two year after their marriage, when child turned to 10 years, then the average age of the family becomes 38 years. What was the age of the daughter in law at the time of marriage ?

Initial total age = 40 * 3 = 120 years. At child's 10th birthday, total family age = 38 * 5 = 190 years. The 70-year increase spans: 10 years for each of 3 original members, 10 years for the child, and 12 years for daughter-in-law from marriage to child's 10th birthday. 70 - 30 - 10 - 12 = 18, so her age at marriage was 18 - 12 = 12 years.