Two identical right-angled trapezoids fit together to form a square. The upper base of this trapezoid is 12 cm, and the lower base is 18 cm. What is the perimeter of the square after joining together?
两个完全相同的直角梯形正好拼成一个正方形，这个梯形的上底是12厘米，下底是18厘米，拼成后的正方形周长是多少厘米．
<gadget id="calculator">12 + 18</gadget>
<output>30</output>

<gadget id="calculator">30 * 4</gadget>
<output>120</output>

<result>120</result>