In April this year, the number of cloudy days in our county accounted for 10% of the whole month, and the number of rainy days accounted for 30% of the whole month. The number of cloudy days is exactly equal to the sum of rainy days and cloudy days. How many cloudy days are there in April?
我县今年四月份阴天天数占全月的10%，雨天天数占全月的30%．多云天数正好等于雨天、阴天天数的和，四月份多云有多少天？
<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">30 * (1/10)</gadget>
<output>3</output>

<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">30 * (3/10)</gadget>
<output>9</output>

<gadget id="calculator">3 + 9</gadget>
<output>12</output>

<result>12</result>