50% of substitute teachers walk out after 1 hour of teaching. 30% of the remainder quit before lunch. If 60 substitute teachers show up at 7 AM, how many will be left after lunch?
Let's be accurate as possible and think first.
21
First find the number of substitute teachers left after half quit: 60 teachers / 2 = 30 teachers.

Then find how many teachers quit after an hour but before lunch: 30 teachers * 30% = 9 teachers.

Then subtract that number from the number of teachers left after one hour to find how many are left after lunch: 30 teachers - 9 teachers = 21 teachers.

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The answer is 21