Q: How many 5 digit numbers can be formed using digits 1,2,3 with exactly one digit repeating 3 times.
Ans: We can choose the digit that repeats in 3c1 ways. The remaining two digits in the 5 digits number can either be same or different. If they are same, there are 2 ways of choosing the digit. In total there would be:
3c1 * ( 2* 5!/(2!*3!) + 5!/(3!)) = 120.
But the answer is 90. Can some one please explain how.