Question:
Tap A and Tap B can fill a tank at a rate of 3 L per minute and 5 L per minute respectively. At first, only Tap A was turned on. When the tank was 1/4-filled with water, Tap B was then turned on. Six minute later, the tank was 7/8-filled with water. How much water was there in the tank just before Tap B was turned on?
Solution:
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