Place 20 chess pieces on the three sides of an equilateral triangle, how many pieces should be placed in total.
在正三角形的三条边上摆棋子，每条边上摆20枚，一共要摆多少枚．
<gadget id="calculator">20 * 3</gadget>
<output>60</output>

<gadget id="calculator">60 - 3</gadget>
<output>57</output>

<result>57</result>