The land area on the earth's surface is about 150 million square kilometers, which is 210 million square kilometers less than the ocean area. How many billion kilometers is the ocean area? (solution by equation)
地球表面的陆地面积大约是1.5亿平方千米，比海洋面积少2.1亿平方千米．海洋面积大约是多少亿千米？(用方程解)
<gadget id="calculator">5.1 + 2.1</gadget>
<output>7.2</output>

<result>7.2</result>