Ben takes 6hrs to get a job done & Ted takes 11hrs to get the same job done. If they work together how long will it take them?
What can be added is the rate of work.
This is how much work each finishes in an hour.
Ben's rate + Ted's rate
= 1/6+1/11
= 17/66
Together they do 17/66 of the work in an hour.
So time it takes to get the work done
= 1/(17/66)
= 66/17
= 3 15/17 hours