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 37-26=11 with both. Just adding up the numbers counted 11 of them twice.
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?
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