If bob can mow a lawn in 2 hours and tim can mow the same lawn in 4 hours, how long will it take if they work together?

I think the answer would be 3 hours because they both do half the lawn the bob would take 1 hour and tim would take 2 and if you add 1 plus 2 then its 3 so I think its 3

Bob can mow 1/2 of the lawn in one hour.

Tim can mow 1/4 of the lawn in one hour.

If they work together for one hour, they can mow 1/2 + 1/4 = 3/4 of the lawn.

Therefore working together, they need 4/3 hours = 1 1/3 hours or 80 minutes.