Betty can mow a lawn in 90 minutes. Melissa can mow the same lawn in 60 minutes. How long does it take for both Betty and Melissa to mow the lawn if they are working together?

Answer 1

75 minutes

Answer 2

Clearly the time working together will be less than for either working alone. 75 cannot be the answer.

You need to find x such that

1/x = 1/90 + 1/60
x = 36

You have to consider how much work each one does in a minute. That is what you can add; not the required time.

Answer 3

90-60=30