Working alone it takes Jerry 9 hrs to mop a warehouse. Ruth can mop the same warehouse in 10 hrs. If they worked together how long would it take them?

Answer 1

Jerry does 1/9 of the work in one hour.
Ruth does 1/10 of the work in one hour.
Together they do 1/9+1/10 = 19/90 of the work in one hour.
How long would it take them together to finish the work ?