Question 13
Example 6. In how many minutes can a tank be filled by three pipes whose diameter are 1 cm, 2 cm and 3 cm? Given that the largest pipe alone can fill it in 21 minutes and the amount of water flowing through the pipe is directly proportional to the square of its diameter.
**Editorial input convention:** All three pipes operate simultaneously at constant rates into an initially empty tank, with no other flows. Use the given 21 minutes for the largest pipe. Enter minutes.
**Editorial input convention:** All three pipes operate simultaneously at constant rates into an initially empty tank, with no other flows. Use the given 21 minutes for the largest pipe. Enter minutes.