Answer:
0.6 of an hour
Step-by-step explanation:
A cold pipe takes 4h hours so we can assume ¼ of our job is done in one hour.
After 3 hours go by with the cold pipe on, we have ¼ of the job left.
Now we have our hot pipe, which takes 6 hours for the entire pool, so we can assume that it has a rate of ⅙ per hour. Now since these two pipes are working together we can add them.
⅙ + ¼ = ⁵/₁₂
Now we divide.
Since at the start 3 hours had gone by with the cold pipe that would have been ¾ of our time so, 1 - ¾ = ¼
¼ ÷ ⁵/₁₂ = ⅗ or 0.6 of an hour
