The cross section of a river dam is a trapezoid with an area of 225 square meters. Its upper base is 12 meters and its lower base is 18 meters. What is the height of this river dam in meters?
某拦河大坝横截面是个面积为225平方米的梯形，它的上底为12米，下底为18米，这拦河大坝的高为多少米．
<gadget id="calculator">12 + 18</gadget>
<output>30</output>

<gadget id="calculator">2 / 30</gadget>
<output>1/15 = around 0.066667</output>

<gadget id="calculator">225 * (1/15)</gadget>
<output>15</output>

<result>15</result>