Two schemes are proposed for stepping up scores on a 500-mark paper. Scheme A adds 6% of the maximum marks to every candidate's score; Scheme B adds 12% of the candidate's own score. For a candidate who scored 240, which scheme adds more, and by how much?

Scheme A adds 6% of 500 = 30 marks, the same for everybody. Scheme B adds 12% of 240 = 28.8 marks. So A adds 30 - 28.8 = 1.2 marks more. The trap is reading 'twice the rate' as 'twice the benefit': B's rate is double A's, but it acts on the candidate's 240 rather than on the maximum of 500, and 240 is less than half of 500. B would only win for a candidate scoring above 250.