A)If the order of the speakers is important, we have 20x19x18x17, or 116,280 different options.
b) If order is not important, then we have 20 choose 4, or 4845 different slates. ☺☺☺☺
I could explain it, but you have to ask me, other than that the answer is 5(x+7)(9x-4).
The answer is B
hope it helps