Xiao Ming buys lychees. It is known that he buys 2 kilograms on the first day, 4 kilograms on the second day, and 5 kilograms on the third day. The total cost is 42 yuan. The price of lychees is 80% of the previous day. How much money can I save in total by buying in three days?
小明买荔枝，已知第一天买2千克，第二天买4千克，第三天买5千克，共花42元钱，荔枝的价格每天是前一天的80%，若这些荔枝全在第三天买，共可节省多少钱？
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">(4/5) * (4/5)</gadget>
<output>16/25 = around 0.64</output>

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

<gadget id="calculator">(16/25) * 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>