A cold water pipe can fill a swimming pool in 4 hours, and a hot water pipe in 6 hours. If cold water pipe is turned on and hot water pipe is turned on 3 hours later then how long will it take them to fill the pool together?

the answer:

0.9

just flip the nine upside down, you have the REAL 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).