Buy a total of 100 stamps of 30 cents and 50 cents, the total price is 39 yuan, how many stamps of 50 cents are bought.
买3角和5角的邮票共100张，总价39元，其中5角的邮票买了多少张．
<gadget id="calculator">39 * 10</gadget>
<output>390</output>

<gadget id="calculator">100 * 3</gadget>
<output>300</output>

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

<gadget id="calculator">5 - 3</gadget>
<output>2</output>

<gadget id="calculator">90 / 2</gadget>
<output>45</output>

<result>45</result>