It costs 4 yuan to buy 2 bottles of mineral water, how much does it cost to buy 15 bottles of mineral water.
买2瓶矿泉水需4元，买15瓶矿泉水需要多少元．
<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<gadget id="calculator">2 * 15</gadget>
<output>30</output>

<result>30</result>