The knowledge of buying shoes: If the shoes are a size, that is, b centimeters, they have such a relationship: a=2b-10. Xiao Ming wants to wear shoes of size 40, that is, how many centimeters should he wear.
买鞋的学问：如果鞋子是a码，也就是b厘米，它们有这样的关系：a=2b-10．小明要穿40码的鞋子，也就是要穿多少厘米的鞋子．
<gadget id="calculator">40 + 10</gadget>
<output>50</output>

<gadget id="calculator">50 / 2</gadget>
<output>25</output>

<result>25</result>