Answer: 6 hours
Step-by-step explanation:
Given : Time taken by Type JQ pump to fill the tank = 72 hours
Then , Time taken by Type JQ pump to fill half of the tank = 36 hours
Time taken by Type JT pump to fill the tank = 18 hours
Then, Time taken by Type JT pump to fill the tank = 9 hours
Now, if he tank starts at half full then the time taken by two Type JQ pumps and a Type JT pump all three pumps working together to fill the tank :-
[tex]\dfrac{1}{t}=\dfrac{1}{36}+\dfrac{1}{36}+\dfrac{1}{9}\\\\\Rightarrow\dfrac{1}{t}=\dfrac{1+1+4}{36}=\dfrac{1}{6}\\\\\Rightarrow t=6[/tex]
Hence, it will take 6 hours to fill the tank.