Solution:
we are given that
The radio station manager will award a free stereo to a contest winner. There are 20 black stereos, 15 white stereos, 10 red stereos, and 5 silver stereos to choose from.
we have been asked to find
The probability that the stereo randomly selected by the contest winner is NOT white?
Total number of stereo=50
Total number of white stereo=15
1 can nbe chosen out of 50 in
ways.
1 can nbe chosen out of 15 in
ways.
So the probability that the chosen sterio is white
So the probability that the chosen sterio isNOT white 
(8/4) = (10/x)
8x = 4 x 10
8x= 40
x = 40/8
x=5
It will take 5 hours.
You divide the answer by the smaller factor, and you must get the answer of the bigger factor.