The size of shoes is usually in "yard" or "centimeter", and the conversion relationship between them is: b=2a-10 (b indicates the number of yards, a indicates the number of centimeters). Teacher Li always wears size 42 shoes, which is how many centimeters.
鞋的尺码通常用"码"或"厘米"作单位，它们之间的换算关系是：b=2a-10(b表示码数，a表示厘米数)．李老师都是穿42码的鞋，也就是多少厘米．
<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">(1/2) * 42</gadget>
<output>21</output>

<gadget id="calculator">21 + 5</gadget>
<output>26</output>

<result>26</result>