[求助]请教prep DS1-10

Of the 800 students at a certain college, 250 students
live on campus and are more than 20 years old. 
How many of the
            800
students live on campus and are 20 years old or less?

(1)  640 students
at the college are more than 20 years old.

(2)  60 students
at the college are 20 years old or less and live off campus.

答案选C

我画出集合图反而没有思路了。求解。