Pipe A fills a tank of capacity 700 liters at the rate of 40

1) 1:+40; 2:+30; 3:-20... 50l/3min
2) 650/50=13; 13*3=39min
3) last round: 650+40(1minute)+10(1/3 of a minute)
4) 39+1+1/3=40 minutes 20 seconds

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Why this process is not working for below?

https://gmatclub.com/forum/pipe-a-and-p ... 80611.html

Pipe A and Pipe B fill water into a tank of capacity 1000 litres at the rate of 100 litres/minute and 25 litres/minute respectively. Pipe C drains water from the same tank at the rate of 50litres/minute. Pipe A is kept open for a minute and then closed, then pipe B is kept open for a minute and then closed, followed by a drain pipe C for a minute and then closed. This process is repeated until the tank is full. How long will it take to fill the tank.

A) 39 mins
B) 40 mins
C) 39 mins and 20 secs
D) 37 mins
E) 41 mins

In three minutes net gain is 100+25-50=75 liters;

After 13 cycles (13*3=39 minutes) net gain will be 13*75=975 liters.

Then to fill 25 liters pipe A will need 25/100=1/4 min.

So total time \(39+\frac{1}{4}\) min = 39 min 15 sec

But Answer is D.
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1. In a cycle of 3 minutes, 40+30-20=50 liters are filled
2. In 13 such cycles, 13*50 = 650 liters are filled.
3. Then in the next fresh cycle, A fills 40 liters and 690 liters are filled.
4. The remaining 10 liters are filled by B in 20 seconds for a total time of 40 minutes 20 seconds
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A = + 40 litres a minutes
B= + 30 litres a minute
C=-20 litres a minute

total transaction in 3 minutes
40+30-20=50 litres

600 litres in 6*6 = 36 minutes
670 litres in 38 minutes
650 litres in 39 minutes
690 in 40 minutes
need 10 more litres which is 1/3rd the rate of B
so 60/3=20 seconds
D

can't we instead first figure out first 630 litres of the tank's capacity would get obtained in what time and add 2 more minutes to it ?

Question: It takes 3 mins (or 1 cycle by A, B, C) to fill 50 liters. How many mins to fill 700 liters (say x)? x = 700 x 3 / 50 = 42 mins?

net flow rate ; 40+30-20 ; 50 ltrs every 3 mins
700 ltrs nearest round value ; 650 ltrs ; in 13*3 ; 39 mins
in 1 min additional ; 40 ltrs from pipe A ; 690 ltrs
and Pipe B ; 10/30 ; 1/3 mins
or say 40 mins * 1/3 * 60 ; 40 mins and 20 sec
IMO D

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