If john finishes his job in 4 hours, and mike finishes the job in 5 hours, how many hours will it take them to finish the job

1/4 = john

1/5 = mike

1/4 + 1/5 = 9/20.

about 2 hours.

(2+2/9 hours to be exact)

John takes 4 hours to finish a job

⇒ John can finish 1/4 of the job in 1 hour

Mike takes 5 hours to finish the job

⇒ Mike can finish 1/5 of the job in 1 hour

If both John and Mike work together

⇒ John and Mike can finish (1/4 + 1/5 = 9/20) of the job in 1 hour

1 hour = 9/20 of the job done

[Multiple 20/9 on both sides]

1 * 20/9 hour = 9/20 * 20/9 job done

20/9 hour = 1 job done

So the time needed is 20/9 hour

20/9 hour = 2 2/9 hours

2 2/9 hours