The No. 4 Primary School in a certain city used 90 tons of water in the first half of last year, and 102 tons of water in the first half of the year before. How many tons of water was saved on average in the first half of last year compared to the first half of the previous year?
某市第四小学去年上半年用水90吨，前年上半年用水102吨．去年上半年平均每月比前年上半年平均每月节约用水多少吨？
<gadget id="calculator">102 - 90</gadget>
<output>12</output>

<gadget id="calculator">12 / 6</gadget>
<output>2</output>

<result>2</result>