SOLUTION: Chris can finish a job in 3 hours and Matt can finish in 9 hours. If Matt works alone for 4 hours and then is REPLACED by Chris who finishes the job alone, how long will it take Ch

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Question 862401: Chris can finish a job in 3 hours and Matt can finish in 9 hours. If Matt works alone for 4 hours and then is REPLACED by Chris who finishes the job alone, how long will it take Chris to finish the job?
The answer: 5/3
I put x/3 + x/9 - x/4 = 1
I know it isn't right, but I only did one problem correctly, i just don't understand it as much.
Please help me.
Please and thank you.
Answer by fcabanski(1391)   (Show Source): You can put this solution on YOUR website!
Chris finishes 1/3 of the job per hour. Matt finishes 1/9 of the job per hour.


The formula is time x rate = portion of the job completed.


Matt works 4 hours, so he completes 4 * 1/9 = 4/9 of the job.


Chris has to complete the job. 99 - 4/9 = 5/9 of the job remaining.


time x rate = portion of the job completed


t * 1/3 = 5/9


t = 5/9 * 3/1 = 5/3 hours.

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