A stationery shop sold 300 notebooks and 200 pens in June. In July its notebook sales rose by 20% while its pen sales fell by 15%. What was the change in the total number of items sold?

July notebooks = $300\times 1.20=360$ and July pens = $200\times 0.85=170$, so the total moves from 500 to 530: a rise of 30/500 = 6%. 'A rise of 5%' subtracts the two rates (20 - 15) and 'a rise of 2.5%' takes their plain average; both ignore that the shop sold half as many pens as notebooks, so the two rates carry unequal weight. 'A rise of 30%' reads the gain of 30 items as a percentage.