The area of a trapezoid is 759 square meters, the length of the lower base is 45 meters, and the height is 23 meters. How many meters is the upper base?
一块梯形的面积是759平方米，下底长45米，高是23米，则上底是多少米？
<gadget id="calculator">23 / 2</gadget>
<output>23/2 = around 11.5</output>

<gadget id="calculator">45 * (23/2)</gadget>
<output>1_035/2 = around 517.5</output>

<gadget id="calculator">759 - (1_035/2)</gadget>
<output>483/2 = around 241.5</output>

<gadget id="calculator">(483/2) / (23/2)</gadget>
<output>21</output>

<result>21</result>