Two inlet pipes P and Q can fill a tank in 10 hours and 12 hours separately. There is an outlet R that can let out water from the tank fully in 30 hours. If all the pipes are opened simultaneously, how much time will it take to fill the tank completely?

Correct option is A

Given:
Pipe P can fill the tank in 10 hours.
Pipe Q can fill the tank in 12 hours.
Outlet pipe R can empty the tank in 30 hours.
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Formula Used:
Work done by all pipes in 1 hour when working together = (Work done by P) + (Work done by Q) - (Work done by R).
Solution:
Work done by P in 1 hour = 1/10
Work done by Q in 1 hour = 1/12
Work done by R in 1 hour = 1/30
Net work done in 1 hour- 1/10 + 1/12 - 1/30 = 9/60 = 3/20
Time taken to fill the tank = 20/3 hours = 6 hours 40 minutes.