If you buy a small exercise book of 1, you still have 1 yuan. Buy 1 large exercise book, lack 1 yuan. Explain that the big one is more expensive than the small one, 2 yuan. Suppose two big ones become small ones, and if you buy one later, you will get eight small ones, leaving five dollars. (2 1-5)÷8=2

2+2=4 Every 2 yuan is small and every 4 yuan is big.