Through the first pipe, the pool can be filled in 9 hours, and through the second in 11 hours.

Through the first pipe, the pool can be filled in 9 hours, and through the second in 11 hours. two pipes were opened for one hour. what part of the pool is left unfilled after that?

Let’s denote the volume of the pool as 1.

We find which part of the pool each of the pipes fills in an hour of work.

We divide the volume of the pool by the time of filling of each pipe.

1/9 = 1/9 part (through 1 pipe).

1/11 = 1/11 part (through 2 pipes).

In this case, if two pipes are open at the same time, then in an hour of work they will fill:

1/9 + 1/11 = 11/99 + 9/99 = 20/99.

From this it follows that the following will remain unfilled:

1 – 20/99 = 99/99 – 20/99 = 79/99 part of the pool.

Answer: After an hour of work, 79/99 will remain.