20. there are 15 members of the show choir. in how many ways can you arrange 4 members in the front row when order does not matter?

The first chair can be any  one of the 15.
   For each of those ...
The second chair can be any one of the remaining 14.
      For each of those ...
The third chair can be any one of the remaining 13.
   For each of those ...
The fourth chair can be any one of the remaining 12.

Number of ways to fill the 4 chairs = (15 x 14 x 13 x 12) = 32,760 .

But ...

Each set of 4 people can be seated in (4 x 3 x 2 x 1) = 24 orders.
So each group of 4 people is represented 24 times among the 32,760.

If the order doesn't matter, you're really asking how many different
groups of 4 people can occupy the front row.

That's        (32,760) / (24) = 1,365 sets of 4 members, in any order.