Final answer:
Using the principle of inclusion and exclusion, we calculated that 30 people saw Movie B by considering the number of people who saw only Movie A and those who saw both movies.
Step-by-step explanation:
To determine how many people saw Movie B, we can use the information given and apply the principle of inclusion and exclusion from set theory. We are told that 40 people saw Movie A, and 20 people saw both Movie A and Movie B. Since 50 people went to see movies in total and some people saw both movies, we can calculate the number of people who saw Movie B by adding the number of people who saw only Movie B to the number who saw both movies.
Let's denote the number of people who saw only Movie B as x. The total number of people who saw either movie or both is given to be 50. So, we have:
Number who saw only A + Number who saw both + Number who saw only B = Total number of people
40 (only A) + 20 (both A and B) + x (only B) = 50
Since the 20 people who saw both movies are already included in the 40 who saw Movie A, we do not need to add them again to find out how many saw Movie A. Therefore, we subtract the 20 from 40 to get the number of people who saw only Movie A:
40 (only A) - 20 (both A and B) + x (only B) = 50
20 + x = 50
Now, we solve for x:
x = 50 - 20
x = 30
Therefore, 30 people saw Movie B.