SOLUTION: Jack usually mows his lawn in 5 hours. Mary can mow the same yard in 7 hours. how much time would it take for them to mow the lawn together? they could mow the lawn in ???? ho

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Question 156779: Jack usually mows his lawn in 5 hours. Mary can mow the same yard in 7 hours. how much time would it take for them to mow the lawn together?
they could mow the lawn in ???? hours if worked together.
Found 2 solutions by phd, edjones:
Answer by phd(3) (Show Source):
You can put this solution on YOUR website!
Consider Jack's rate as lawn per hour and Mary's rate as lawn per hour. Together they have a rate of . Thus in one hour they can mow 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 which is 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 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 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.

Answer by edjones(8007) (Show Source):
You can put this solution on YOUR website!
Jack does 1/5 job in 1 hr.
Mary does 1/7 job in 1 hr.
1/5 + 1/7
=7/35 + 5/35 = 12/35 of job is done in 1 hr together
35/12 hrs it takes them both to do the job or 2 hrs 11/12 hrs or 2 hrs 55 min.
.
Ed