Question

how many 4-digit numbers are there with no digit repeated​

Answer 1

$9\cdot9\cdot8\cdot7=4536$

first digit - any except for 0 (hence 9)

secong digit - any of remaining 8 (plus 0, hence 9 again)

third digit - any of remaining 8 (hence 8)

fourth digit - any of remaining 7 (hence 7)