The price of Mickey’s toothpaste is 15 yuan per box, but the sales volume is not large. In order to promote sales, the store advertises heavily and sells at a reduced price. Later, the sales volume doubles, and the income increases instead (3/5). How much is the reduction in a box of toothpaste? ?
米奇牙膏价是每盒15元，但销量不大，为了促销，商店大做广告，而且降价销售，后来销量增加2倍，收入反而增加了(3/5)，问一盒牙膏降低了多少元？
<gadget id="calculator">2 * 15</gadget>
<output>30</output>

<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">30 - 15 - 9</gadget>
<output>6</output>

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<result>3</result>