Question

One crew can seal a parking lot in 9 hours and another in 12 hours. how long will it take to seal the parking lot if the two crews work together?

Answer 1

first crew = 1 lot in 9 hours, therefore 1 hour =
`(1)/(9)` of the lot.
second crew = 1 lot in 12 hours, therefore 1 hour =
`(1)/(12)` of the lot.
In one hour, both can seal (
`(1)/(9) + (1)/(12)`) of the lot, which is
`(7)/(36)`


`(7)/(36)` = 1 hour

`(1)/(36)` =
`(1)/(7)` hour

`(36)/(36)` = 5
`(1)/(7)` hour

So they need 5
`(1)/(7)` hour if they work together.