A distributor keeps 50% of a consignment in his main godown, 40% of the main godown's stock on its ground floor, and 30% of the ground-floor stock in a cold room. The cold-room stock is what per cent of the whole consignment?

Take the consignment as 100. Main godown = 50, ground floor = 40% of 50 = 20, cold room = 30% of 20 = 6. So 6%, which is just $0.50\times 0.40\times$ 0.30. 12% is $0.40\times 0.30$ - it forgets the first stage. 15% is 30% of 50, skipping the middle stage. 120% adds the three rates, which is meaningless: each rate is taken OF the stage before it, so they multiply.