A merchant surveyed 400 people to determine the way they learned about an
upcoming sale. The survey showed that 180 learned about from radio, 190 from
television, 190 from the newspaper, 80 from radio AND television, 90 from
radio AND newspapers, 50 from television AND newspapers, and 30 from ALL THREE
sources.

U = all people surveyed (n(U) = 400.)
R = those who learned about the sale by radio (n(R) = 180)
T = " " " television (n(T) = 190)
N = " " " newspapers (n(N) = 190)

Also:

n(R intersect T) = 80

n(R intersect N) = 90

n(N intersect T) = 50

n(N intersect R intersect T) = 30

From this information and the key idea, all basic regions can be filled in.