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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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