The average age of all the members of a family is 25 yr. The average age of males is 30 yr while the average age of females is 20 yr. If the number of females in the family is 15, then find out the number of males.

20
30
15
18

Correct Answer: 15

Explanation:

Let number of males = y
Then, total number of family members = 15 + y
Sum of age of family members = sum of age of males + sum of age of females
25(15 + y) = 30y + 15x20
⇒ 375 + 25y = 30y + 300
⇒ 5y = 75
∴ y = 15

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