72 People on a Bus — Apply the Procedure

72 people are on a bus. At a stop, some get off and 15 get on — leaving 68. How many got off? The trap is option B (4), which comes from simply subtracting 72 − 68 = 4 and ignoring the 15 who got on. The correct method adds the 15 back: 72 + 15 − 68 = 19, or equivalently, work backwards from 68.

Bus/queue change problems — where a starting quantity changes by two steps (one unknown, one known) to reach a final quantity — are among the most common very-easy working-backwards questions in OC MR. They appear at the start of the paper and are designed to be solved quickly with a single equation.

The examiner tests whether students can translate a two-step change (subtract unknown, add known) into an equation and solve for the unknown. Option B (4) is the deliberate trap for students who compute start − end = 72 − 68 = 4 while ignoring the 15 who boarded.

A starting total is given. An unknown amount is removed, then a known amount is added (or vice versa). A final total is stated. Students must set up: start − unknown + known = end, then solve for the unknown.