SOLUTION: Tom and Jerry have a mowing business. It takes Tom 30 minutes to mow a yard and it takes Jerry 40 minutes to mow the same yard. If they work together, how long will it takes them t

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Question 578104: Tom and Jerry have a mowing business. It takes Tom 30 minutes to mow a yard and it takes Jerry 40 minutes to mow the same yard. If they work together, how long will it takes them to mow the yard?
I was thinking along the lines of Average rate between them is 35 minutes and if both of them are mowing it will be half of that which is 17.5 minutes.
The answer in the back of the book states that it will take 120/7 minutes. I have no idea how they got these figures? Any assistance would be appreciated.
Answer by josmiceli(19441) (Show Source):
You can put this solution on YOUR website!
Add their rates of working to get
rate of working together
Tom's rate: ( 1 yard ) / ( 30 min )
Jerry's rate: ( 1 yard ) / ( 40 min )
Let = time in minutes to mow yard working together
Their rate working together: ( 1 yard ) / ( t min )
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Multiply both sids by

min
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All these type of problems are about adding ( sometimes subtracting ) rates