Subtract 41 1/2 minutes from 60 minutes, and then divide the result by 1/2 minute.
  60            =>        59 2/2
- 41 1/2      =>       -41 12
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                                18 1/2 minutes worth of commercials.
Divide 18 1/2 by 1/2;
37
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2
                37        2
-------- =  ------ * ----- = 37     There were 37-1/2-minute commercials.  ;(
 1               2        1
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2