Column calculation: 15 plus 40% of a number, the sum is 30, what is this number?
列式计算：15加上一个数的40%，和是30，这个数是多少？
<gadget id="calculator">30 - 15</gadget>
<output>15</output>

<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">15 / (2/5)</gadget>
<output>75/2 = around 37.5</output>

<result>75/2 = around 37.5</result>