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
Answer: 2 2/9 hours
