If we restrict our attention to workers who like to drink beer, the probability that a randomly selected blue-collar worker prefers to drink light beer is 0.2 or 20%.
How the probability is determined:
The number of workers who like to drink beer = 250
The number of blue-collar workers who like to drink beer = 150
The number of blue-collar workers who like light beer = 50
The number of blue-collar workers who prefer regular beer = 100
The number of white-collar workers who like to drink beer = 100
The number of white-collar workers who prefer light beer = 50
The number of white-collar workers who prefer regular beer = 50
Thus, the probability that a randomly selected blue-collar worker who prefers to drink light beer = 50/250 = 0.2 or 20%.
Question Completion:
The number of workers who like to drink beer = 250
The number of blue-collar workers who like to drink beer = 150 (light beer = 50 and regular beer = 100)
The number of white-collar workers who like to drink beer = 100 (light beer = 50 and regular beer = 50)