Tsingtao beer at the beverage counter in the supermarket is 3.6 yuan per bottle. The National Day promotion launched a "buy three get one free" promotion. Mom even bought 24 bottles of Tsingtao beer. Please do the math, how much money did mom save in this promotion?
超市饮料柜台青岛啤酒每瓶3.6元，国庆促销开展"买三送一"的优惠活动，妈妈连买加送共得到青岛啤酒24瓶．请你算一算，妈妈在这次优惠活动中节省了多少钱？
<gadget id="calculator">3.6 * 24</gadget>
<output>86.4</output>

<gadget id="calculator">3.6 * 3</gadget>
<output>10.8</output>

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">24 / 4</gadget>
<output>6</output>

<gadget id="calculator">10.8 * 6</gadget>
<output>64.8</output>

<gadget id="calculator">86.4 - 64.8</gadget>
<output>21.6</output>

<result>21.6</result>