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
A circle is of radius 8 units. Then the probability that a randomly chosen point inside the circle is
closer to the centre than the circumference, is