Answer 1

There are 6 different digits and we have to form 4-digit numbers, i.e., n = 6, r = 4.

If the repetition of digits is not allowed, then the number of 4-digit numbers

=  ⁶P₄

= `(6!)/(2!)`

= `(6 xx 5 xx 4 xx 3 xx 2!)/(2!)`

= 360.