Can someone answer. there is one problem. there's a picture. you!

The combination formula nCr=n!(n−r)!(r!)

n is the number of elements (total). 

r is the number of elements picked from the set.

In this case, n = 15 and r = 4

15!(11!)(4!)

=1365

So, there are 1365 ways of listening to the CD's.

Hope I helped!

proofs involving isosceles triangles often require special consideration because an isosceles triangle has several distinct properties that do not apply to normal triangles.(more about triangle types) therefore, when you are trying to prove that two triangles are congruent, and one or both triangles, are isosceles you have a few theorems that you can use to make your life easier.

step-by-step explanation:

an isosceles triangle has two congruent sides and two congruent angles. the congruent angles are called the base angles and the other angle is known as the vertex angle. ∠bac and ∠bca are the base angles of the triangle picture on the left. the vertex angle is ∠abc