A tank fitted with two pipes is to be filled with water. one pipe can fill it in 15 hours. after it has been open for 3 hours the second pipe is opened and the tank is filled in 4 hours how long would it take the second pipe alone to fill the tank

In an attempt to be environmentally friendly, the city council decides that there must be at least 18 trees on each block in the city. a sample of 20 blocks was randomly surveyed, with these results (number of trees per block): 24, 17, 7, 20, 28, 27, 10, 21, 30, 8, 30, 27, 22, 27, 27, 13, 27, 20, 24, 17. what is the sample mean number of trees per block ? a. 17.7 b. 21.3 c. 24 d. there is insufficient data to answer this question.