Four times a number minus 80% of that number gives a difference of 25% of 6.4. Find this number.
一个数的4倍减去这个数的80%，差是6.4的25%．求这个数．
<gadget id="calculator">1.6 / 3.2</gadget>
<output>0.5</output>

<result>0.5</result>