A histogram is to be drawn for the distribution 0–10: 12, 10–20: 20, 20–40: 32, 40–70: 24 (class interval: frequency). The classes have unequal widths, so the bars must be drawn to make AREA represent frequency, taking a width of 10 as the standard. What height should the bar for the class 20–40 be given?

The adjusted height is (standard width ÷ class width) × frequency = $(10÷20)\times 32=16$. Drawing the bar at its raw frequency 32 would give it an area of $32\times 2=64$ standard units, twice the frequency it is supposed to show — the usual mistake with unequal classes. The value 1.6 is the frequency per unit of 1 rather than per unit of 10, and 8 is the adjusted height of the last class, $(10÷30)\times 24$.