The original price of a certain toothpaste is 15 yuan a box. In order to promote sales, the price is lowered, the sales volume is doubled, and the income is increased by three-fifths. How much is the price of a box of toothpaste reduced?
某种牙膏原价15元一盒，为了促销，降低了价格，销量增加了二倍，收入增加了五分之三，则一盒牙膏降价多少元．
<gadget id="calculator">3 / 5</gadget>
<output>3/5 = around 0.6</output>

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

<gadget id="calculator">15 + 9</gadget>
<output>24</output>

<gadget id="calculator">24 / 3</gadget>
<output>8</output>

<gadget id="calculator">15 - 8</gadget>
<output>7</output>

<result>7</result>