Shandong Jinxiang is the hometown of garlic in China. During the 13 years from 1989 to 2002, the garlic planting area increased from 107 square kilometers to 341 square kilometers. A head of ordinary garlic weighs about 32 grams, and the largest head of garlic produced in Jinxiang weighs 640 grams. How many square kilometers does Jinxiang's garlic planting area increase on average every year?
山东金乡是中国的大蒜之乡，从1989年到2002年的13年间，大蒜种植面积由107平方千米增加到341平方千米．一头普通大蒜约重32克，金乡产出的最大一头大蒜重640克．金乡的大蒜种植面积平均每年增加多少平方千米？
<gadget id="calculator">341 - 107</gadget>
<output>234</output>

<gadget id="calculator">234 / 13</gadget>
<output>18</output>

<result>18</result>