There is a circular park of radius 5 meters. Three friends Farida, Somaiya and Farid are sitting at equal distance on its boundary each having a toy telephone (connected using strings) in their hands to talk each other. Find the length of the string between a pair of the telephones.

In Farid's class, 12 students play hockey, and 15 students play volleyball. There are 6 students who play both games. How many students are there in the class?