Hello, ayushdadhwal!

We have:

. . .

In how many ways we can put the letters of the word LETTER in the boxes,

so that no row is empty and each box contains not more than one letter.

In the first row, place a letter.

There are 6 choices for the letter and 3 choices for its position.

. . There are ways.

In the second row, place a letter.

There are 5 choices for the letter; 1 choice for its position.

. . There are ways.

In the third row, place a letter.

There are 4 choices for the letter; 3 choices for its position.

. . There are ways.

In the fourth row, place a letter.

There are 3 choices for the letter; 1 choice for its position.

. . There are ways.

In the fifth row, place a letter.

There are 2 choices for the letter, 3 choices for its position.

. . There are ways.

So, there are: . ways

. . to place one letter in each row.

For the last letter, there 6 choices of boxes.

Hence, there are: . ways

. . to place the six letters in the boxes.

But there is duplication caused by

. . the two identical L's and two identical E's.

Therefore, there are: . ways

. . to place the letters of the word LETTER in the boxes.