Answer:
0.6 hours
Step-by-step explanation:
Let's say the swimming pool is 12,000 gallons.
The flow rate of the cold water pipe is:
12000 gal / 4 hr = 3000 gal/hr
The flow rate of the hot water pipe is:
12000 gal / 6 hr = 2000 gal/hr
If the cold water pipe is turned on for three hours, the volume of water is:
3000 gal/hr × 3 hr = 9000 gal
That leaves 3000 gallons. The time to fill the rest of the pool with both the cold and hot water pipes is:
3000 gal / (3000 gal/hr + 2000 gal/hr) = 0.6 hr
It takes 0.6 hours (36 minutes).