Grandma used a fence to surround a rectangular vegetable field with a length of 15 meters and a width of 10 meters. How many meters of fence does grandma need at least?
奶奶用篱笆在墙边围了一个长方形菜地，长15米，宽10米，奶奶至少要用多少米的篱笆？
<gadget id="calculator">10 * 2</gadget>
<output>20</output>

<gadget id="calculator">15 + 20</gadget>
<output>35</output>

<result>35</result>