We have to find the possible number of ways in which we can post seven letters among four letter boxes when repetition of distribution of letters is allowed.

We will use the concept of multiplication because there are seven sub jobs dependent on each other and are performed one after the other.

The thing that is distributed is considered to have choices, not the things to which we have to give them; it means that in this problem the letters have choices more precisely four choices are there for each letter and letter boxes won’t choose any because letters have the right to choose.

The number of ways in which we can post letters among four letter boxes where repetition of distribution is allowed 4 × 4 × 4 × 4 × 4 × 4 × 4 = 4⁷.