Street sweeping vehicles can clean 3 miles of streets per hour. a city owns two street sweepers, and each sweeper can be used for three hours before it comes in for an hour to refuel. during an 18-hour shift, how many miles of street can be cleaned?

We know that
In a period of 4 hours (3 hours of work and 1 hour to refuel)
each street sweepers clean> 3 miles*3 =9 miles

divide 18 hours by 4
18/4=4.5
4.5 is equal to 4 periods of 4 hours plus 2 hours

Multiply 4 by 9 miles
4*9=36 miles

in the period of 2 hours (in this period there is no refuel)
3*2=6 miles

each street sweepers clean in 18 hours> (36+6)> 42 miles
two street sweepers clean in 18 hours=2*42> 84 miles

the answer is
84 miles

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