A kind of book is priced at 21 yuan each, and the profit can be 40% after sale. How much profit can you make if you sell it at 20% off the price?
一种书每本定价21元，售后可以获利40%，如果按定价的八折出售可获利多少元？
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">21 * (4/5)</gadget>
<output>84/5 = around 16.8</output>

<gadget id="calculator">40 / 100</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">1 + (2/5)</gadget>
<output>7/5 = around 1.4</output>

<gadget id="calculator">21 / (7/5)</gadget>
<output>15</output>

<gadget id="calculator">(84/5) - 15</gadget>
<output>9/5 = around 1.8</output>

<result>9/5 = around 1.8</result>