30 Secs Approach

Let's assume the pool capacity is 100 liters. 1/4 of the capacity is 25 liters.

Assume the pool is filled with 25 at time t. at t+1 the pool will be filled with 50 and at t+2 it will be filled with 100.

In other words, from 1/4 capacity it takes 2 hours to reach full capacity.

0----------------------------------25 liters-----------50 liters------------100 liters

0h----------------------------------6h---------------------------------------8h

Answer is D.

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