A kind of computer is priced by the merchant at a profit of 30% of the cost. In order to attract customers, it is sold at a "10% discount". As a result, each computer still makes a profit of 680 yuan. How much does this computer cost per unit?
一种电脑，商家按成本的30%赢利定价，为了吸引顾家，又以"九折"卖出，结果每台电脑仍获利680元．这种电脑成本每台多少元？
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">1 + (3/10)</gadget>
<output>13/10 = around 1.3</output>

<gadget id="calculator">90 / 100</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">(13/10) * (9/10)</gadget>
<output>117/100 = around 1.17</output>

<gadget id="calculator">(117/100) - 1</gadget>
<output>17/100 = around 0.17</output>

<gadget id="calculator">680 / (17/100)</gadget>
<output>4_000</output>

<result>4_000</result>