Naughty Dad has just recharged some money in the stored-value card of his mobile phone. If he spends 6 yuan a day, he can use it for 15 days. If he saves 1.5 yuan a day, how many days can this stored-value card be used for?
淘气的爸爸手机储值卡中刚刚充了一些钱，如果每天消费6元，能使用15天，如果每天节约1.5元，这张储值卡可以使用多少天？
<gadget id="calculator">6 - 1.5</gadget>
<output>4.5</output>

<gadget id="calculator">15 / 4.5</gadget>
<output>3.333333</output>

<gadget id="calculator">6 * 3.333333</gadget>
<output>19.999998</output>

<result>19.999998</result>