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a and b can finish a job in 3 hours, b can finish 7 hours, how long does it take for a to finish the job

Guest Mar 9, 2017
 #1
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1/3 = 1/a + 1/b

1/3 = 1/a + 1/7

1/3-1/7 = 1/a

7/21 -3/21 = 1/a

4/21 = 1/a

a= 21/4 = 5 1/4 hr

ElectricPavlov  Mar 9, 2017

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