You know that initially there were 8 pipes to fill the drum. Out of which, 2 were damaged, and only one of the two pipes were retreived back to usable form. What will be the number of pipes that can be used to fill the drum now?
7
The number of pipes that are damaged is 2. Hence, the number of usable pipes will be 8-2 = 6
Now, out of the 2 damaged pipes, 1 was retreived back to usable form. Hence, the total number of usable pipes will now be 7.