For 4 h and 48 min. The first pipe fills the whole pool for 8 hours so from here we see that it fills 1/8 each hour. The second pipe fills it for 12 hours so it fills 1/12 each hour. Now both of them together fill 1/8 + 1/12 of the pool each hour. That equals: 1st pipe 3/24 of the pool each hour and the 2nd pipe 2/24 of the pool each hour. So, the two pipes together fill 3/24 + 2/24 each hour. That's 5/24 of the pool each hour.
1st hour: 5/24
2nd hour: 10/24
3rd hour: 15/24
4th hour: 20/24
And we need 4/24 more. So, if 5/24 is 1 h, then 1/24 is (60 minutes/5) 12 minutes. We need 4*1/24 and that is 12 minutes * 4 and that is 48 minutes.
So, they will fill the pool for 4 h and 48 min.
