



 
Hi Bob, This is a question for which I would never use a formula. Formulas are much too easy to forget. For this and many similar problems I just remember to look at rates. You mow the lawn in 30 minutes so your rate is 1 thirtieth of a lawn per minute (1/30 lawns/min). It takes Joe 3 times as long so it takes him 90 minutes and thus his rate is one ninetieth of a lawn per minute (1/90 lawns/min). Hence if you work together you can work at
and lawns/minute. But you want the number of minutes per lawn which is the reciprocal of lawns per minute so it will take you
Exactly what the formula says! Harley  


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