Question

How many different numbers of six digits each can be formed from the digits 4, 5, 6, 7, 8, 9 when repetition of digits is not allowed?

Answer 1

Number of ways of filling the first digit = 6
Number of ways of filling the second digit = 5 (as repetition is not allowed)
Number of ways of filling the third digit = 4
Number of ways of filling the fourth digit =3
Number of ways of filling the fifth digit = 2
Number of ways of filling the sixth digit = 1
Total numbers = 6$\times$5$\times$4$\times$3$\times$2$\times$1 = 720