This year, the theme of the Shanghai World Expo is "Better City, Better Life". At present, the urban population accounts for about (2/5) of the 1.3 billion population in my country, and the proportion of the general developed countries is about (7/10). To reach the level of the general developed countries, how many billion more will my country's urban population have to increase?
今年，上海世博会的主题是"城市让生活更美好"．目前，我国13亿人口中城市人口约占(2/5)，一般发达国家这一比例约为(7/10)．要达到一般发达国家的水平，我国城市人口还要增加多少亿？
<gadget id="calculator">7 / 10</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">(7/10) - (2/5)</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">13 * (3/10)</gadget>
<output>39/10 = around 3.9</output>

<result>39/10 = around 3.9</result>