I got this tip from a guide on this forum:
Permutations Including like Objects. Formula: n! / r¹! r²! .....
where r¹ is the number of one object which are alike and r² is the number of a second object alike, and so on.
There is no choosing.
The order of the objects is important,
You have some objects which are the same.
You want to arrange 10 books on a shelf but you have 2 copies of one book and 3 copies
of another. The number of ways you can arrange them is 10! / 2! x 3! which equals 3,628,800 ÷ 2 x 6 = 302,400.
It is so so sooooo great, I have just kept it in my log!
GMAT Club Premium Membership - big benefits and savings