Correct Answer  Option 4 : 50 min
Given:
A can fill up the tank in 60 min
B can fill the tank in 75 min
C empties the tank in 100 min
Concept used:
Work = time × efficiency
Calculation:
Work 
Time 
Efficiency 
A 
60 mins 
300/60 = 5 
B 
75 mins 
300/75 = 4 
C 
100 mins 
300/100 = 3 
work (LCM of time) 
300 units 

Efficiency of C = 3 as it is emptying the pipe.
Total efficiency of A, B ,and C together = 5 + 4  3 = 6
Time to fill the tank = 300/6
⇒ 50 mins
∴ Time taken to fill the tank is 50 mins.