Xiaohong has a box of chocolates. If she eats 6 yuan a day, she can still have 4 yuan left. If she eats 5 yuan a day for the first 3 days, and then eats 7 yuan a day, she will just finish eating. The unit price of each piece of chocolate is 2.5 yuan. How much do you need?
小红有一盒巧克力，如果每天吃6块，还可以剩4块，如果前3天每天吃5块，以后每天吃7块，则正好吃完，每块巧克力单价2.5元，则这盒巧克力需要多少元．
<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<gadget id="calculator">3 * 5</gadget>
<output>15</output>

<gadget id="calculator">21 - 15</gadget>
<output>6</output>

<gadget id="calculator">4 + 6</gadget>
<output>10</output>

<gadget id="calculator">7 - 6</gadget>
<output>1</output>

<gadget id="calculator">10 / 1</gadget>
<output>10</output>

<gadget id="calculator">10 * 6</gadget>
<output>60</output>

<gadget id="calculator">60 + 4</gadget>
<output>64</output>

<gadget id="calculator">64 * 2.5</gadget>
<output>160</output>

<result>160</result>