The urban population accounts for about 40% of my country's 1.3 billion population, and this proportion is about 70% in developed countries. To reach the level of the general developed countries, how many billion more will my country's urban population have to increase?
我国13亿人口中城市人口约占40%，一般发达国家这一比例约为70%．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">70 / 100</gadget>
<output>7/10 = around 0.7</output>

<gadget id="calculator">40 / 100</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>