A bookshelf can accommodate 6 books from left to right. If 8 identical books on each of the languages A, B, C and D are available, in how many ways can the bookshelf be filled such that books of the same language are not put adjacent to each other?
The number of ways to put 5 identical balls in 3 identical boxes such that no box is empty, is
There are 3 letters and 3 directed envelopes. Write the number of ways in which no letter is put in the correct envelope.