Time To Drain The Reservoir

Here is the question! 

Let T be the capacity of the tank. Let L be the large pipe and S be the small pipe.

T/6L = 12

L = T/72

T/(3L + 9S) = 8

Putting L = T/72,

S = T/108

Hence T/5S = 108/5 gives S = 21.6 hours.

To conclude, small pipes require 21.6 hours i.e. 21 hours and 36 minutes to drain the reservoir.