store uses 240 tons of water in the first half of the year and 180 tons of water in the second half of the year. Calculated at 2 yuan per ton of water, how much is the average monthly water saving in the second half of the year compared to the first half?
一个商店上半年用水240吨，下半年用水180吨．每吨水按2元计算，下半年比上半年平均每月用水节约多少元？
<gadget id="calculator">240 - 180</gadget>
<output>60</output>

<gadget id="calculator">60 / 6</gadget>
<output>10</output>

<gadget id="calculator">10 * 2</gadget>
<output>20</output>

<result>20</result>