SOLUTION: Working alone Mandy can complete a project in 7 hours. It would take Colin 3 hours to complete the same project. If they work together, how long will it take them to complete the p

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Question 892418: Working alone Mandy can complete a project in 7 hours. It would take Colin 3 hours to complete the same project. If they work together, how long will it take them to complete the project?
Answer by josmiceli(19441) About Me  (Show Source):
You can put this solution on YOUR website!
Ad their rates of working
Mandy's rate:
( 1 project ) / ( 7 hrs )
Colin's rate:
( 1 project ) / ( 3 hrs )
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Let +t+ = time to finish project
if they work together
+1%2F7+%2B+1%2F3+=+1%2Ft+
Multiply both sides by +21t+
+3t+%2B+7t+=+21+
+10t+=+21+
+t+=+2.1+ hrs
Convert +.1+ hrs to min
+.1%2A60+=+6+ min
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Working together, it will take
them 2 hrs and 6 min