Grandpa Wang used a rope to measure the surface area of a round table. He measured it like this: if the rope is wrapped around the table for 3 weeks, the remaining rope will be 212cm; if the rope is wrapped around the table for 4 weeks, the rope will be short of 102cm. Do you know how Grandpa Wang measured the area of the round table top? What is the area of the table?
王爷爷用一根绳子测量一张圆桌桌面的面积，他是这样测量的：如果用绳子绕桌边3周，绳子余下212cm；若用绳子绕桌边4周，绳子则缺102cm．你知道王爷爷是怎样测量出圆桌桌面的面积的吗？这个桌面的面积是多少呢？
<gadget id="calculator">314 / 3.14 / 2</gadget>
<output>50</output>

<gadget id="calculator">50 ** 2</gadget>
<output>2_500</output>

<gadget id="calculator">3.14 * 2_500</gadget>
<output>7_850</output>

<result>7_850</result>