There are 30 students, so we are looking for the group which contains the \((30 + 1) \div 2 = 15\frac{1}{2}th\) value, which lies halfway betwen the 15th and the 16th value. The cumulative frequency ...

When class widths are not equally sized, they are called unequal class intervals. The area of the bar represents the frequency, so to find the height of the bar, divide frequency by the class width.