How many 3letter sequences are there if no characters can be

How many 3-letter sequences are there if no characters can be repeated? I need a step by step solution.

Solution

for 3 letter sequence can be calculated as follows:

in 1st place you can fill all 26 chars.

In 2nd place we can fill remaining 25 chars.

in the remaining last place we can fill 24 chars that are left.

Hence total no of sequence are 26*25*24 = 15600