Two pipes fill the pool in 3 hours 45 minutes, and one first pipe fills the pool in 10 hours.

Two pipes fill the pool in 3 hours 45 minutes, and one first pipe fills the pool in 10 hours. How many hours does one second pipe fill the pool?

Let x be the capacity of the first pipe, then y is the second pipe. The volume of the pool (work performed) is taken as a unit. Let’s compose and solve the equation:
Let’s translate into hours:
3 hours 45 minutes = 3.75 hours
1 / x + 1 / y = 1 / 3.75
By condition 1 / x = 1/10, where x = 10. Substitute in the first equation:
1/10 + 1 / y = 1 / 3.75
3.75y – 10y = -3.75
y = 6
Answer: one second pipe will fill the pool in 6 hours.