Place chess pieces on the three sides of an equilateral triangle, one at each of the three vertices, 20 pieces on each side, how many chess pieces are needed in total.
在正三角形的三条边上摆棋子，三个顶点各摆一枚，每边摆20枚，一共需要多少枚棋子．
<gadget id="calculator">20 * 3</gadget>
<output>60</output>

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

<result>57</result>