The average age of a group of 20 men is 30 years. A 50 year old man leaves the group, while a woman joins the group. The average age decreases by 1 year. What is the age of the woman ?

30 years

ATQ,

Sum of age of 20 men = 20 × 30 = 600

Sum of age of the group when old man leaves = 600 - 50 = 550

Average age of the group when a woman joins = 29

Sum of age of new group = 29 × 20 = 580

Age of the woman = 580 - 550 = 30 years