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kevincan
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Step by step solution, a little bit of math to do.
Let's find the rate of both, do the rate of Farad will be 1/5 while the rate of Saud will be 1/8. Now we know that Farad started x minutes before or x/60 hour before Saud joined him. The work has been completed 13/4 h after Farad started. It means that Farad worked alone for x/60 hours and that both worked for a total time of 13/4-x/60 hours. Now translating in math it means:

1=1/5*x/60 + (1/5+1/8)*(13/4-x/60)

Solving for x we have x=27
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Wow ! Your first post ! Welcome to Gmatclub !
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Let total work be 40 units. Hence, F rate = 8 and S rate = 5.

F worked for 13/4 hours and S worked for (13/4 - x/60) = (195-x)/60 hours

Work done by F + Work done by S = 40 units.
(8 * 13/4) + 5 * (195-x)/60 = 50
26 + (195-x)/12 = 40
(195-x)/12 = 14
195-x = 168
Therefore, x = 27.
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