A tank is emptied everyday at a fixed time point. Immediately thereafter, either pump or pump or both start working until the tank is full. On Monday, alone completed filling the tank at . On Tuesday, B alone completed filling the tank at . On Wednesday, alone worked till , and then B worked alone from to , to fill the tank. At what time was the tank filled on Thursday if both pumps were used simultaneously all along?
A tank is emptied everyday at a fixed time point. Immediately thereafter, either pump or pump or both start working until the tank is full. On Monday, alone completed filling the tank at . On Tuesday, B alone completed filling the tank at . On Wednesday, alone worked till , and then B worked alone from to , to fill the tank. At what time was the tank filled on Thursday if both pumps were used simultaneously all along?
Solution
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 of tank per hour
B fills of tank per hour
Combined rate:
Rate = of tank per hour
Time to fill together:
Time = 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!