In a class of 40 students, 15 watched television last night, 32 did homework, 9 ate pizza, 11 watched television and did homework, 7 watched television and ate
pizza, 4 ate pizza and did homework and 3 did all three (Note: When we say
11 students watched television and did homework, some of those students may
have also eaten pizza. Likewise, for the other numbers.)
How many students did none of these activities?