Discrete Structures

Prove: Among any set of 21 whole numbers there must be 3 that are the same How many permutations of letters {A, B, C, D, E, F, G, H} contains a substring BDEG. Prove it by writing down the permutations. Write down the 3 permutation of set {A, B, C, D, E, F, G, H} and tell how many 3 permutation it has