math
posted by stacey .
in a class of 30 students, 17 have computers at home and 20 have cassettes. If 4 students in the class have neither a computer or a cassette, how many of the 30 students have both?

If you count all the students with computers, and then count all those with cassettes, you count twice the ones that have both. So, adjust the total accordingly.
30 students. 4 with nothing, leaves 26 with either computer or cassette or both
17+20 = 37, but there are only 26 students available, so there must be 3726=11 with both. Just adding up the numbers counted 11 of them twice.