The profit of a certain tape recorder is 30% of the purchase price. It is known that its retail price is 390 yuan per set. How much is the cost of this tape recorder per set?
某种录音机的利润是进价的三成，已知它的零售价是每台390元，求这台录音机的成本是每台多少元？
<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">390 / (13/10)</gadget>
<output>300</output>

<result>300</result>