The original price of a certain toothpaste is 15 yuan per box, but the price was lowered for promotion, so the sales volume increased by 2 times, and the income increased (3/5). Q: How much is the price reduction for a box of toothpaste?
某种牙膏原价15元一盒，为了促销，降低了价格，因此销量增加了2倍，收入增加了(3/5)．问：一盒牙膏降价多少元？
<gadget id="calculator">1 * 15</gadget>
<output>15</output>

<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

<gadget id="calculator">1 * 15 * (3/5)</gadget>
<output>9</output>

<gadget id="calculator">45 - 15 - 9</gadget>
<output>21</output>

<gadget id="calculator">21 / 3</gadget>
<output>7</output>

<result>7</result>