You should consider the RANGE of the data, not the count. Use the minimum and maximum values to decide on your six classes.
Question:
The following ordered set of numbers represents the salinity of 30 specimens of water taken from a stretch of sea, near the mouth of a river.
4.2 , 4.5 , 5.8 , 6.3 , 7.2 , 7.9 , 8.2 , 8.5 , 9.3 , 9.7 , 10.2 , 10.3 , 10.4 , 10.7 , 11.1 , 11.6 , 11.6 , 11.7 , 11.8 , 11.9 , 12.4 , 12.4 , 12.5 , 12.6 , 12.9 , 12.9 , 13.1 , 13.5 , 14.3
(a) Form a grouped frequency table for the data using six equal classes.
(b) Draw a cumulative frwquency graph. Estimate the 12th highest salinity level. Calculate the percentage error in this estimate.
Attempt:
(a) As they wanted 6 classes and the total number of specimens is 30. I though of makeing 6 classes with 5 specimens each but the answer in the book is totally different!
How can I create a Frequency Table?