Question

How many four digit number can be formed with the 10 digits 0, 1, 2,……9 if no number can start with 0 and if repetitions are not allowed?

Answer 1

At the first place, 9 digits can come excluding 0.

At the second place, 9 digits can come excluding the 1^st digit (as repition is not allowed).

At the third place, 8 digits can come (as repition is not allowed).

At the fourth place, 7 digits can come (as repition is not allowed).

Thus, number of four digit numbers which can be formed with the 10 digits 0, 1, 2,……9

Number of 'four digit number' = 9 × 9 × 8 × 7 = 4536