Region A: £3.9m over 15 stores. Region B: £2.0m over 9 stores. What were the higher region's sales per store?

Convert each series to the same per-unit rate (value ÷ count) before comparing — never compare raw totals. The correct answer is £260k. Traps to avoid: £433k comes from the "cross divide" error; £222k comes from the "absolute not per store" error; £133k comes from the "cross divide" error; £246k comes from the "pooled" error.