Question 972950
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Lone Ranger: 100% of crime/8 hours=12.5% of crime per hour
Tonto: 100% of crime/12 hours=8.33% of crime per hour
If Tonto starts at 9 AM, at 11 AM there is (100% of crime-(2 hrs)(8.33%/hr))=
100%-16.66%=83.33% of the crime left. 
Together, the two clean up 12.5%+8.33%=20.83% per hour
83.33%/20.83%=4 hours. After the Lone Ranger joins Tonto (at 11 am), it will take them 4 hours to clean up the remaining 83.33% of the crime. Four hours from 11 AM is 3PM, so they will be finished at 3 PM.