企业生产一种产品需顺次经过四个车间，这四个车间的废品率分别为1.5%、2.0%、2.0%和1.0%。该企业生产这种产品的平均废品率是多少？

The number of phone calls arriving at a switchboard each hour has been recorded and the following frequency distribution has been developed.
What is the approximate range of the number of phone calls arriving each hour？（）

The number of phone calls arriving at a switchboard each hour has been recorded and the following frequency distribution has been developed.
What is the cumulative frequency of the third class？（）