I need help with a simple word problem.

John can rake lawn in 30 minutes. Todd can rake the same lawn in 60 minutes. how fast can they rake it together?

I got a rate per unit of time: together they can rake 1/20 of the lawn per minute. Now how do I get the total minutes it will take them to rake the lawn together? the answer is 20 minutes, but how do I get to that?

John's rate: 1/30 lawn/minute

Todd's rate: 1/60 lawn/minute

together: 1/30 + 1/60 = 1/20 lawn/minute

1 lawn / ( 1/20 lawn/minute) = 20 minutes

rate = job/time

time = job/rate

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