Find the number of ways in which four distinct balls can be kept into two identical boxes so that no box remains empty.

Answer

7

Explanation

4 distinct balls can be divided into two nonempty groups as 1,3 or 2,2
Sine boxes are identical, number of ways of division and distibution are same
`therefore` Number of ways `=(4!)/(1!3!)+(4)/(2!2!2!)=4+3=7`.