Question 156779
Consider Jack's rate as {{{1/5}}} lawn per hour and Mary's rate as {{{1/7}}} lawn per hour. Together they have a rate of {{{1/5 + 1/7 = 12/35}}}. Thus in one hour they can mow {{{12/35}}} of the lawn. Now the rate times the time it takes them will give you one whole lawn. So, since they are mowing just one lawn take the reciprocal of {{{12/35}}} which is {{{35/12}}} and this is how many hours it takes them to mow a lawn together. Now this might be easier to see in minutes. If we divided 35 by 12 we get 2 with 11 left over. So {{{ 2 (11/12)}}} hours. Thinking of a clock which has 12 numbers. The 11 on a clock represents 55 minutes. Another way of looking at that is that {{{11/12 = 55/60}}} and there are 60 minutes in one hour. Therefore the answer is 2 hours and 55 minutes which is less time than it take either of them alone.