Here's the first part . . .
There are: . possible orders for the 15 phones.Fifteen telephones has just received at a service center.
Five are cellular , five are cordless , and five conventional corded phones .
(a) If phones are randomly selected for service, what is the probability
. . .that all the cordless phones are serviced first?
There is one way for the -phones to be first.
The other 10 phones can follow in: . ways.
Hence, there are: . ways for the -phones to be first.
I'm still working on the second part . . .