i didn't understand this part

It is given that the inlet pipe can fill a tank in 6 hours but because of a leak it took 40 more minutes to fill the tank.

Now, here we can say,
Work done by the inlet in 40 minutes = Work done by the leak in 6 hours
Work done by the inlet in 40/60
hour = Work done by the leak in 6 hours
Work done by the inlet in 2/3
hour = Work done by the leak in 6 hours
Since, the inlet pipe fill the tank in 6 hours, so to get 6 in LHS we will multiply both
sides by 9
Work done by the inlet in 2/3 × 9 hour = Work done by the leak in 6 × 9 hours
Work done by the inlet in 6 hours = Work done by the leak in 54 hours
Hence, the leak will take 54 hours to empty the tank when inlet pipe is not operating.

E is the answer

The best method to solve this is:
When the tank had the leakage it took 6 hours to fill that is 360 minutes
And when the leakage was repaired it only took 40 minutes.

Now we will simply take LCM of 360 and 40 which is 360 for that we need to multiply 1 for 360 and 9 for 40 so that they both become 360

From this we can know that the Time taken to empty the tank will be nothing but :

(360 * 9) => This will give you answer in minutes.
If you convert it to hours just divide it by 60 and you get the answer as 54 hours