A bus depot ran 200 trips last year, of which 180 were on time. This year it ran 230 trips, of which 207 were on time. What happened to the depot's on-time percentage?

180/200 = 90% and 207/230 = 90%, so the share is exactly where it was. Both the on-time count and the trip count grew by the same 15% (180 to 207 and 200 to 230), and a fraction whose top and bottom are multiplied by the same figure does not move. 'It rose by 15%' quotes the growth in the on-time count, which is a count and not a share. The two 'percentage points' options assume a share must shift whenever both figures shift; it shifts only when the two grow at different rates.