IMAT 2017 Q60 [Ordering]

How many ways are there to order the letters ‘AABBC’? (For example, ‘ACABB’ and ‘AABBC’ are two ways.)

A. 5
B. 120
C. 60
D. 116
E. 30


Firstly, we will take the letters and distinguish same letters with subscripts, like A_1A_2B_1B_2C.
That gives 2+2+1=5 different symbols that can be permuted in 5! ways.
If we erase the subindices, this means e.g. A_1A_2, are now the same. This means that to get the total ways we need to divide 5! with 2!2!1!, so the total is:

\frac{5!}{2!2!1!}=\frac{5\cdot4\cdot3\cdot2\cdot1}{2\cdot1\cdot2\cdot1\cdot1}= 5\cdot3\cdot2=5\cdot6 =30.

So the correct answer is (E) 30.