=In a class of 30, 20 have computers. 17 have cassettes. 4 have nothing. How many have both?
first subtract the 4 that have nothing from the total class of 30 to find how many have something.
Now, in a class of 26, 20 have computers and 17 have cassettes, how many have both?
Add the two groups 20 + 17 = 37
since there are only 26 class members, 37-26=11, 11 class members must have both.
check this answer:
4=nothing
9 =computers only
6 =cassettes only
11 =have both
4+9+6+11=30