three. There are 5 students in Class 1. During the Spring Festival, each two students will send each other a greeting card. How many greeting cards should they prepare in total?
三．1班有5个同学，在春节期间每两人互送一张贺卡，他们一共要准备多少张贺卡？
<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<result>20</result>