Every laptop returned to a repair center is classified according its needed repairs: (1) LCD screen, (2) motherboard, (3) keyboard, or (4) other. A random broken laptop needs a type i repair with probability pi = 24-i /15. Let Ni equal the number of type i broken laptops returned on a day in which four laptops are returned.
(a) Find the joint PMF
(b) What is the probability that two laptops require LCD repairs?
(c) What is the probability that more laptops require motherboard repairs than keyboard repairs?