A school records the daily time (in hours) that a group of students spent on the internet during one week. The data are grouped into unequal class intervals before drawing a histogram.
(i) What is meant by "frequency density" with reference to a grouped frequency distribution?
(ii) Explain, with calculations, why the heights of the bars for the following two classes must differ even though the second class has a larger frequency.
Class A: 0–1 h (12 students)
Class B: 1–3 h (18 students)