The area of the outer circle of a ring is 400 square meters, and the area of the inner circle is 75% of the outer circle. What is the area of the circle in square meters?
一个圆环外圆面积是400平方米，内圆面积是外圆的75%，环形面积是多少平方米．
<gadget id="calculator">75 / 100</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">400 * (3/4)</gadget>
<output>300</output>

<gadget id="calculator">400 - 300</gadget>
<output>100</output>

<result>100</result>