A pipe can fill a tank in 20 minutes and another pipe can fill the tan...
Let's assume that the tank has a capacity of 120 units (LCM of 20, 40, and 15).
The first pipe can fill the tank in 20 minutes, so in 1 minute, it can fill 6 units (120 units divided by 20 minutes).
The second pipe can fill the tank in 40 minutes, so in 1 minute, it can fill 3 units (120 units divided by 40 minutes).
The third pipe can empty the tank in 15 minutes, so in 1 minute, it can empty 8 units (120 units divided by 15 minutes).
After 5 minutes, the first pipe has filled 5 * 6 = 30 units.
After 5 minutes, the second pipe has filled 5 * 3 = 15 units.
So, after 5 minutes, the tank has 30 + 15 = 45 units filled.
When all three pipes are open, the net filling rate of the tank is (6 + 3) - 8 = 1 unit per minute.
To empty the tank, we need to remove 45 units (as calculated above).
So, it will take 45 minutes to empty the tank with all three pipes open.
Therefore, the correct answer is option (B) 75.
A pipe can fill a tank in 20 minutes and another pipe can fill the tan...
E