3. A store bought a batch of greeting cards at 21 cents each, and sold them for a total of 14.57 yuan. If each card was sold at the same price, and the selling price did not exceed twice the purchase price, then the store made a profit of $\qquad$yuan.
【Key Points】Number Theory, Prime Factorization
【Difficulty】ふ
【Answer】 4.7 yuan
【Analysis】14.57 yuan $=1457$ fen, $1457=31 \times 47$
The selling price of each item does not exceed twice the purchase price, 47 is the number of items, 31 fen is the selling price; the store made a profit of $(31-21) \times 47=470$ (fen) $=4.7$ yuan.