The upper base of a trapezoid is 4 meters, the height is 5 meters, and the area is 30 square meters, so how many meters is the lower base?
一个梯形上底4米，高5米，面积30平方米，那么下底多少米．
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">30 * (2/5)</gadget>
<output>12</output>

<gadget id="calculator">12 - 4</gadget>
<output>8</output>

<result>8</result>