In how many ways 100 persons can be divided into 50 pairs ?

In how many ways 10 persons can be divided into 5 pairs?

In how many ways 10 persons can be divided into 5 pairs?

In how many ways 5 persons can be seated in a row ?

In how many ways 100 persons can be arranged along a row so that two specified persons must always be separated by exactly 7 of them.

In how many ways 4 persons can be arranged in 3 rows with out any restriction. If each row can accommodate any number of persons.