The average age of boys and girls in a class is 10.5 yr; that of the boys is 10.6 yr and that of the girls is 10.1 yr. If there are 60 boys in the class, how many girls are there in the class?

By alligation rule, Boys Girls 10.6 10.1 10.5 10.5-10.1 : 10.6 - 10.5 0.4 : 0.1 4 : 1 Therefore, if there are 60 boys, then there are 15 girls(=60/4).

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