In 2023 a library had 2,500 members, 36% of whom were students. In 2024 the membership rose to 3,000 and the number of student members rose to 990. What happened to the students' share of the membership?

In 2023 students numbered 36% of 2,500 = 900. In 2024 they were 990 out of 3,000 = 33%. So the share FELL by 3 percentage points even though the number of students rose by 90. The trap is to reason from the count alone: student numbers grew 10% (900 to 990) but total membership grew 20% (2,500 to 3,000), and a share falls whenever the part grows more slowly than the whole. 'Rose by 10 percentage points' quotes the students' own growth rate as though it were a share.