Discussion on Bangladesh Mathematical Olympiad (BdMO) National
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 $A$.)
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 .
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