A piece of rectangular colored paper is 21 cm long and 15 cm wide. After cutting out the largest square, what is the remaining area?
一张长方形彩纸长21厘米，宽15厘米，先剪下一个最大的正方形之后，余下的面积是多少？
<gadget id="calculator">21 - 15</gadget>
<output>6</output>

<gadget id="calculator">15 * 6</gadget>
<output>90</output>

<result>90</result>