Ning Ning spent 6 yuan and 80 cents to buy 10 greeting cards and postcards, each greeting card was 80 cents, and each postcard was 50 cents. How many postcards did Ningning buy?
宁宁花了6元8角钱买贺卡和明信片共10张，贺卡每张8角，明信片每张5角．宁宁买了多少张明信片．
<gadget id="calculator">8 * 10</gadget>
<output>80</output>

<gadget id="calculator">80 - 68</gadget>
<output>12</output>

<gadget id="calculator">8 - 5</gadget>
<output>3</output>

<gadget id="calculator">12 / 3</gadget>
<output>4</output>

<result>4</result>