permutation and combination

The natural numbers that satisfy the condition are,

1, 5, 9, 13, 17, 21.

Hence there are 6 ways to select first number.

5 ways to select second.

4 ways to select third.

3 ways to select fourth.

2 ways to select fifth.

Hence the number of ways = ⁶P₅ = 6x5x4x3x2 = 720

Regards,

Team,

TopperLearning.