Betty can mow a lawn in 20 minutes. Bullwinkle can mow the same lawn in 60 minutes. How long does it take for both Betty and Bullwinkle to mow the lawn if they are working together? Express your answer as a reduced fraction.

  15 minutes

Step-by-step explanation:

Betty mows at 3 times the speed that Bullwinkle does, so is equivalent to having 3 Bullwinkles in her place. That makes the lawn get mowed as though 4 Bullwinkles were working, so it will take 1/4 the time it takes Bullwinkle to mow the whole yard. 1/4 of 60 minutes is 15 minutes.

Working together, Betty and Bullwinkle will take 15 minutes to mow the lawn.