SOLUTION: Chris can construct a robot on an erector set in 7 hours. His friend, Cristobal, can construct a robot using the same materials in 8 hours. How long would it take them to constru

Question 433597: Chris can construct a robot on an erector set in 7 hours. His friend, Cristobal, can construct a robot using the same materials in 8 hours. How long would it take them to construct a robot if they both worked together?
Answer by stanbon(75887) (Show Source): You can put this solution on YOUR website!
Chris can construct a robot on an erector set in 7 hours. His friend, Cristobal, can construct a robot using the same materials in 8 hours. How long would it take them to construct a robot if they both worked together?
----
Chris rate = 1/7 job/hr
Friend's rate = 1/8 job/hr
---
Together rate = 1/x job/hr
--------------------------------
Equation:
rate + rate = together rate
1/7 + 1/8 = 1/x
Multiply thru by 56x to get:
8x + 7x = 56
15x = 56
x = 3 11/56 hrs (time to do the job together)
---
x = 3 hr 12 minutes
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Cheers,
Stan H.
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