Question

It takes 8 hours for one crew to clean an office building. It takes a second crew 10 hours to clean the same building. If the first crew works 2 hours and leaves and the second crew takes over to complete the job, how long will it take the second crew to finish cleaning the building?

Answer 1

1/8-1 hour
2/8- 2 hours

2/8+1/10x=1
1/10x=6/8
x=7.5 hrs

Answer 2

let the work to be done be 1 unit

rate of first crew = 1/8
job done by first crew in 2 hours = 2(1/8) = 1/4
job left to be done = 1 - 1/4 = 3/4

rate of second crew = 1/10
time needed for 2nd crew to finish
= (3/4) / (1/10)
= 30/4 hrs or 7.5 hrs