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A tank is emptied everyday at a fixed time point. Immediately thereafter, either pump AA or pump BB or both start working until the tank is full. On Monday, AA alone completed filling the tank at 8pm8 \mathrm{pm}. On Tuesday, B alone completed filling the tank at 6pm6 \mathrm{pm}. On Wednesday, AA alone worked till 5pm5 \mathrm{pm}, and then B worked alone from 5pm5 \mathrm{pm} to 7pm7 \mathrm{pm}, to fill the tank. At what time was the tank filled on Thursday if both pumps were used simultaneously all along?

Solution

✅ Correct Option: 2

We need to find when both pumps working together will fill the tank. Let us break this down using the information from each day.


Monday: Pump A alone fills the tank, finishing at 8pm

Tuesday: Pump B alone fills the tank, finishing at 6pm

Wednesday: Pump A works until 5pm, then pump B works from 5pm to 7pm to complete the job

The key insight is that B finishes 2 hours earlier than A when working alone.


From Wednesday's scenario, we can find exactly how long each pump takes:

A worked for some time, then B worked for exactly 2 hours (5pm to 7pm)

Whatever work A couldn't finish in those hours, B completed in exactly 2 hours

Here's the crucial reasoning: If A had continued working instead of stopping at 5pm, it would have taken A exactly 3 more hours to finish (since B finished the remaining work in 2 hours, and we know A is slower than B).

This means:

A's remaining work = 3 hours of A's work = 2 hours of B's work

So A and B work in the ratio 2:3 (B is faster)

Since B finishes 2 hours earlier than A:

If A takes 6 hours, B takes 4 hours

Difference = 6 - 4 = 2 hours


Since A finishes at 8pm and takes 6 hours:

Tank emptying time = 8pm - 6 hours = 2pm

Let us verify with Tuesday: B finishes at 6pm and takes 4 hours

Tank emptying time = 6pm - 4 hours = 2pm


Individual rates:

A fills 16\frac{1}{6} of tank per hour

B fills 14\frac{1}{4} of tank per hour

Combined rate:

Rate = 16+14=212+312=512\frac{1}{6} + \frac{1}{4} = \frac{2}{12} + \frac{3}{12} = \frac{5}{12} of tank per hour

Time to fill together:

Time = 1÷512=125=2.41 \div \frac{5}{12} = \frac{12}{5} = 2.4 hours = 2 hours 24 minutes


Starting at 2pm (emptying time) + 2 hours 24 minutes = 4:24pm

Key Learning: When pumps work in sequence like Wednesday, the work distribution gives us the ratio of their rates. This ratio method is much faster than setting up complex equations!

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