Question

A blogger who is just starting out made three videos. So far, she has 312 subscribers, who watched one, two, or all three videos. The three videos got 740 total views all together. Assuming each subscriber only viewed a particular video once, how many more people watched all three videos than only one video?

Answer 1

Let a, b, c represent the numbers of subscribers who watched 1, 2, and 3 videos, respectively.
.. a +b +c = 312 . . . . . . . there are 312 subscribers, total
.. a +2b +3c = 740 . . . . there were 740 total views

Subtract double the first equation from the second.
.. (a +2b +3c) -2(a +b +c) = (740) -2(312)
.. c -a = 740 -624 = 116

116 more people watched all three videos than watched one video.