An isosceles trapezoid has a perimeter of 72 cm, a waist of 15 cm, and an upper base of 18 cm. How long is its base in centimeters?
一个等腰梯形的周长是72厘米，腰长15厘米，上底长18厘米．它的下底长多少厘米？
<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<gadget id="calculator">72 - 30 - 18</gadget>
<output>24</output>

<result>24</result>