A rectangle has a perimeter of 40 meters. It is known that the ratio of length to width is 3:1. What is the area of this rectangle in square meters?
一个长方形周长是40米．已知长与宽的比是3：1．这个长方形的面积是多少平方米？
<gadget id="calculator">40 / 2</gadget>
<output>20</output>

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">20 * (3/4)</gadget>
<output>15</output>

<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<gadget id="calculator">15 * 5</gadget>
<output>75</output>

<result>75</result>