This is a chug-plug formula given in my book :
1.The number of ways in which mn different items can be divided equally int m groups, each containing n objects and the order of the groups is not important,is : $\frac {(mn)!}{(n!)^m} \frac {1}{m!} $
2.The number of ways in which mn different items can be divided equally int m groups, each containing n objects and the order of the groups is important,is : $\frac {(mn)!}{(n!)^m} \frac {1}{m!} * (m!) $
I am having some troubles in understanding the above two formulas and hence making mistakes while solving problems based on the same, so I am very inquisitive to know the proof.