The pool was filled with water in 6 hours by opening two pipes. If you open only the first pipe

The pool was filled with water in 6 hours by opening two pipes. If you open only the first pipe, then the pool will be filled in 15 hours. How long does it take to fill the pool if you open only the second pipe?

Let the second pipe fill a full pool in X hours.
Let all the work, the entire pool be designated as a unit (1). Then the first pool filled 1/15 of the pool in 1 hour, and the second pipe filled 1 / X of the pool in 1 hour, then together they filled 1/6 of the pool in 1 hour.
Let’s compose and solve the equation:
1/15 + 1 / X = 1/6;
Bring the whole equation to a common denominator of 30X:
2X + 30 = 5X;
30 = 5X – 2X;
30 = 3X;
X = 30: 3;
X = 10.
Thus, the second pipe filled the entire pool in 10 hours.
Answer: in 10 hours.