If a PSP is sold at the original price, the profit margin is 30%. If the purchase price is reduced by 10%, and the profit margin is 50%, then each PSP will have an extra profit of 300 yuan. How much is the original price of this PSP.
一款PSP若按原价卖出，利润率是30%．如果进价降低10%，并且50%的利润率卖出，那么每台PSP就将多得300元的利润．这款PSP原价是多少元．
<gadget id="calculator">30 / 100</gadget>
<output>3/10 = around 0.3</output>

<gadget id="calculator">300 * (3/10)</gadget>
<output>90</output>

<gadget id="calculator">300 + 90</gadget>
<output>390</output>

<gadget id="calculator">50 / 100</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">10 / 100</gadget>
<output>1/10 = around 0.1</output>

<gadget id="calculator">(1/10) * (1/2)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">(1/2) - (1/10) - (1/20) - 1 - (3/10)</gadget>
<output>-19/20 = around -0.95</output>

<gadget id="calculator">1 + (-19/20)</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">390 / (1/20)</gadget>
<output>7_800</output>

<result>7_800</result>