Install street lights on one side of a road, install one street light at the head and end of the road, install a street light every 20 meters in the middle, and install a total of 169 street lights. How long is this road in meters?
在一条公路的一边安装路灯，路头和路尾各安装一根路灯，中间每隔20米安装一根路灯，一共安装了169根路灯．这条公路长多少米．
<gadget id="calculator">169 - 1</gadget>
<output>168</output>

<gadget id="calculator">20 * 168</gadget>
<output>3_360</output>

<result>3_360</result>