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Bunuel
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GudduMishra
In 1 hour, the inlet pump working alone would fill up 1/10 of the total volume of the tank.
In 1 hour, the outlet pump working alone would empty 1/7 of the total volume of the tank.
When both the pumps are working simultaneously, the total work done will be (1/7-1/10) = 3/70
Since the outlet pump rate was higher, 3/70 of the tank is empty and (1- 3/70) = 67/70, is the remaining volume to be emptied.
So, 1/7 work by the outlet pump was done in 1 hour.
67/70 work will take = 1/(1/7) * 67/70 = 7 * 67/70 = 6.7 hours.

Option D

Pretty good answer, imo. Will just add one additional step of calculation for more context:

We know, w = r x t

1 = (1/7) * t

Therefore, t = 7 hours.

Since it takes 7 hours to empty 1 full tank, to finish 67/70 of the tank, it will take 7 * (67/70) = 6.7 hours.
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Bunuel
Working alone at their respective constant rates, an outlet pump can empty a tank in 7 hours when the tank is full, and an inlet pump can fill the same tank in 10 hours when the tank is empty. Beginning with the full tank. Both the pumps work simultaneously at their respective rates for one 1hour before the inlet pump is shut off. How many more hours would it take the outlet to empty the tank?

(A) 6.1
(B) 6.3
(C) 6.5
(D) 6.7
(E) 6.9



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The inlet pump fills 1/10 of the tank during that hour, and it will take 7/10 of an hour for the outlet pump to get that additional water out. Without this extra water, the outlet pump would have to work for six more hours, so in total it will have to work for 6.7 hours after the inlet pump is turned off. Therefore, the answer is (D) 6.7.
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