Math Class IX Chapter 6 - Sequence and Series

Permutation of n different objects is a subset of given objects? Number of signals that can be given by 5 flags of different colours using 3 flags at a time is 120? Five boys and four girls can be seated on a bench so that the boys and the girls can occupy alternate seats in 20? Number of diagonals that can be formed by joining the vertices of 10 sided figure is 35?