Question

Max can mow a lawn in 45 minutes. Jan takes twice as long to mow the same lawn. If they work together, how long will it take them to mow the lawn?

Answer 1

Max can mow a lawn: 1/45
jan can mow a lawn: 1/90

together they can mow a lawn: 1/(1/45+1/90)
= 1/(1/30)
= 30 min it will take them to mow the lawn.

Answer 2

Max mows a yard in 45 minutes and Jan takes twice as long mowing the same yard. If Max and Jan both work together, it will take approximately 30 minutes to for them both to mow the yard. 

It take Max 1 minute to mow 1/45 of the yard.
It takes Jan 1 minute to mow 1/90 of the yard.

1/45+1/90 =3/90 = 1/30 of the yard can be mowed by both in just one minute. This is how to get the answer, 30 minutes.