When a large number of lychees were on the market in Shenzhen, the daily price was 80% of the previous day. My mother bought 2 kilograms on the first day, 4 kilograms on the second day, and 5 kilograms on the third day, and spent a total of 42 yuan. I buy lychees on the third day, how much can I save?
深圳荔枝大量上市时，每天的价格都是前一天的80%，妈妈第一天买了2千克，第二天买了4千克，第三天买了5千克，共花了42元，如果这些荔枝都在第三天买，能少花多少钱？
<gadget id="calculator">1 * 2</gadget>
<output>2</output>

<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">(4/5) * 4</gadget>
<output>16/5 = around 3.2</output>

<gadget id="calculator">(4/5) * (4/5) * 5</gadget>
<output>16/5 = around 3.2</output>

<gadget id="calculator">2 + (16/5) + (16/5)</gadget>
<output>42/5 = around 8.4</output>

<gadget id="calculator">42 / (42/5)</gadget>
<output>5</output>

<gadget id="calculator">2 + 4 + 5</gadget>
<output>11</output>

<gadget id="calculator">5 * (4/5) * (4/5) * 11</gadget>
<output>176/5 = around 35.2</output>

<gadget id="calculator">42 - (176/5)</gadget>
<output>34/5 = around 6.8</output>

<result>34/5 = around 6.8</result>