Question

Write all the possible 2-digit numbers using the digits 5,7 and 1. Once only in a number​

Answer 1

1,2,5,7,8 and 9. Thus, we have 6 numbers. Then we want to form two-digit numbers without repetition. There are 6 number that is possible to occupy first digit, right? 1,2,5,7,8 or 9. Because it’s without repetition, so the number that have been filled the first digit cannot be the second digit. Thus, the numbers that possible to occupy second place are 5 digits. Thus, there are 6 x 5 = 30 numbers that can we form from those 6 numbers: 1,2,5,7,8 and 9 without repetition.

or by using permutation, there are two place filled by six numbers, so the permutation is 6P2 = 6!/ (6–2)! = 6!/4! = 6 x 5 = 30.