Answer:
60 minutes.
Step-by-step explanation:
Find the time for filling the tank with the first 2 taps:
1/30 + 1/40 = 1/x
Multiply through by the LCD 120x:
4x + 3x = 120
x = 120 /7
= 17.14 minutes.
When the other tap is emptying the tank we have
1/17.14 - 1/ 24 = 1/ x
0.05834 - .04167
= 0.01667
so x = 1 / 0.01667
= 60 minutes to the nearest minute.