UPSC CDS 1 2021 Math Paper

the ratio of boys and girls in the class is 3 : 1 , let the number of boys and girls in the class are 3k and k. Given that the average score of the class is p and average score of the boys is (p+1) , let the average score of the girls is , then total score of the class
⇒(4k)×p=3k(p+1)+k(x)
⇒x=p−3