BITSAT 2019 Paper

(a) Total number of 4-digit numbers
= 5 x 5 x 5 x 5 = 625
(as each place can be filled by anyone of the numbers 1, 2, 3, 4 and 5)
Numbers in which no two digits are identical
= 5 x 4 x 3 x 2 = 120 (i.e. repetition not allowed)
(as 1st place can be filled in 5 different ways, 2nd place can be filled in 4 different ways and so on)
Number of 4-digits numbers in which at least 2 digits are identical
= 625 - 120 = 505