In this situation, 20 m/s is obtained by applying the relevant motion concept carefully. The question tests whether the learner can distinguish the required quantity from related ideas and use the stated information without adding unsupported assumptions. The alternatives 10 m/s, 36 m/s, and 72 m/s do not satisfy the governing definition, graph interpretation, direction convention, or numerical relationship in this case. In examinations, identify the known quantities, keep SI units consistent, select a positive direction where needed, and apply the appropriate definition or equation. This also checks that the answer has a sensible magnitude and unit.