A store has four open checkout stands. Find the number of different ways that six customers can line up at these stands.

The way I did this (stars and bars):

Since there are 4 stands, I need to insert 3 dividers among these 6 people to make 4 groups.

Thus, I have 6+3=9 slots and 3 dividers to place.

9 choose 3 is 84.

Where am I wrong?

Other answers I have tried: 21 (84/4), 120 (10 choose 3), 165 (11 choose 3), 35 (7 choose 3)

Guest Mar 15, 2020