Math Olympiad-5

[Mosammat Arifa Akter]

5. There are some boys and girls in a class. Every boy is friends with exactly three girls, and every girl is friends with exactly three boys. If there are 13 boys in the class, how many girls are there? (Assume that friendship is mutual, i.e. if A is friend of B then B is also friend of A.)

Answer: Given there are 13 boys . Every boy have 3 girls as their friends . And every girl have 3 boys as their friends . Therefore , making one girl as the friend of 3 boys , the total number of girls , as friend of boys are 13*3 or . Taking one girl for three times .So the number of girls is 13 also .