A classmate went to a bookstore for a work-study program in the summer. According to the contract, the bookstore would give him a set of books and 800 yuan after working for 30 days. 500 yuan, how much is this set of books worth?
某同学暑期去书店勤工俭学，按合同规定，干满30天，书店将给他一套书和800元钱，但他只工作了20天，因有事终止了合同，书店给了他一套书和500元钱，这套书值多少钱？
<gadget id="calculator">800 / 30</gadget>
<output>80/3 = around 26.666667</output>

<gadget id="calculator">500 / 20</gadget>
<output>25</output>

<gadget id="calculator">(80/3) - 25</gadget>
<output>5/3 = around 1.666667</output>

<gadget id="calculator">1 / 20</gadget>
<output>1/20 = around 0.05</output>

<gadget id="calculator">1 / 30</gadget>
<output>1/30 = around 0.033333</output>

<gadget id="calculator">(1/20) - (1/30)</gadget>
<output>1/60 = around 0.016667</output>

<gadget id="calculator">(5/3) / (1/60)</gadget>
<output>100</output>

<result>100</result>