A bus depot's yearly note records that the share of its trips run to time rose from 40 in every hundred to 50 in every hundred, the depot having run exactly the same number of trips in each of the two years. Which of the following must be true?

Take a hundred trips in each year. Forty were punctual in the first year and fifty in the second, so the punctual trips went up by ten on a base of forty — a rise of a quarter. The gap of ten in every hundred is a gap in shares, not a rise of a tenth, so the first option confuses the two; a half would need sixty punctual trips; and the late trips went from sixty to fifty, a fall of one sixth and not a quarter.