For a trapezoid, the upper base and height remain the same, the lower base increases by 4 decimeters, and the area increases by 15 square decimeters. What is the height of the trapezoid in decimeters?
一个梯形，上底与高都不变，下底增加4分米，面积增加15平方分米，梯形的高是多少分米．
<gadget id="calculator">2 / 4</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">15 * (1/2)</gadget>
<output>15/2 = around 7.5</output>

<result>15/2 = around 7.5</result>