The cover of a math book is 21 cm long and 15 cm wide, what is the circumference in cm?
数学书的封面长21厘米，宽15厘米，周长是多少厘米．
<gadget id="calculator">21 + 15</gadget>
<output>36</output>

<gadget id="calculator">36 * 2</gadget>
<output>72</output>

<result>72</result>