Students are playing games on the playground. Everyone forms a square with exactly 15 people on each side. How many students are playing games in total?
同学们在操场上玩游戏，大家围成一个正方形，每边正好有15人，一共有多少个同学在玩游戏．
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

<gadget id="calculator">60 - 4</gadget>
<output>56</output>

<result>56</result>