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?
First find the number of substitute teachers left after half quit: 60 teachers / 2 = 
<gadget id="calculator">60/2</gadget>
<output>30</output>
30 teachers.
Then find how many teachers quit after an hour but before lunch: 30 teachers * 30% = 
<gadget id="calculator">30*30*.01</gadget>
<output>9</output>
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 = 
<gadget id="calculator">30-9</gadget>
<output>21</output>
21 teachers.

<result>21</result>