The average score of a class of boys and girls in an examination is A. The ratio of boys and girls in the class is 3 : 1. If the average score of the boys is A + 1, the average score of the girls is-

Answer: Option B

Let the number of boys and girls in the class be 3x and x respectively.
Let the average score of the girls be y.
Then,
⇒ 3x (A + 1) + xy = (3x + x) A
⇒ 3 (A + 1) y = 4A
⇒ y = A - 3