Xiao Ming buys a copy of review materials, and the price is 6.4 yuan after a 20% discount. How much is the material cheaper?
小明买一本复习资料，打八折后是6.4元，则资料便宜了多少元．
<gadget id="calculator">80 / 100</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">6.4 / (4/5)</gadget>
<output>8</output>

<gadget id="calculator">8 - 6.4</gadget>
<output>1.6</output>

<result>1.6</result>