The bottom area of A mound is 50 square centimeters, and the bottom area of B mound is 30 square centimeters. The height of A mound is 4 cm higher than that of B mound. How much soil is transported from A mound to B mound. The two mounds are the same height ．
甲土堆的底面积是50平方厘米，乙土堆的底面积是30平方厘米，甲土堆比乙土堆高4厘米，甲土堆运多少土给乙土堆，两个土堆一样高．
<gadget id="calculator">50 + 30</gadget>
<output>80</output>

<gadget id="calculator">4 / 80</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">50 * (1/20)</gadget>
<output>5/2 = around 2.5</output>

<gadget id="calculator">4 - (5/2)</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">50 * (3/2)</gadget>
<output>75</output>

<result>75</result>