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?

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.

90-60=30

Well, if Betty takes 90 minutes to mow the lawn and Melissa takes 60 minutes, I'm guessing if they work together, they'll do it in no time. With two strong mowers like them, I wouldn't be surprised if they finish mowing the lawn before their lawnmower even breaks a sweat!

To find out how long it takes for Betty and Melissa to mow the lawn together, you can use the concept of work rates. The work rate is the amount of work done per unit of time.

First, let's determine the work rate of each person. Betty can mow the lawn in 90 minutes, so her work rate is 1 lawn / 90 minutes, which can be simplified to 1/90 lawns per minute. Similarly, Melissa can mow the lawn in 60 minutes, so her work rate is 1 lawn / 60 minutes, or 1/60 lawns per minute.

To find their combined work rate, you need to add up their individual work rates. So, the combined work rate of Betty and Melissa working together is (1/90 + 1/60) lawns per minute.

To find out how long it takes for them to mow the lawn together, you need to flip the combined work rate and divide it into 1 (representing 1 lawn). This gives you:

1 / (1/90 + 1/60) = 1 / (3/180 + 2/180) = 1 / (5/180) = 180 / 5 = 36 minutes.

Therefore, it takes Betty and Melissa 36 minutes to mow the lawn together.

75 minutes