A single bar 30 cm long represents all 60 students of a class and is divided in proportion to how they travel to school: 24 by bus, 18 by cycle, 12 on foot and 6 by car. How long, in cm, is the part of the bar standing for the students who walk?

The 30 cm bar stands for 60 students, so one student is worth 30 ÷ 60 = 0.5 cm. The 12 who walk therefore need $12\times 0.5=6$ cm; the same answer comes from noticing that they are 12/60 = 1/5 of the class and taking 1/5 of 30 cm. Turning the scale upside down — using 60 ÷ 30 = 2 students per centimetre as a multiplier — gives 24 cm, which is longer than the bus section and so cannot be right. The value 12 is a head count, not a length, and 2.5 comes from dividing 30 by 12.