I bought a total of 100 pencils with 30 cents and 50 cents, and the total cost was 39 yuan. How many pencils with 50 cents were bought?
买3角和5角的铅笔共100支，共用去了39元，买了5角的铅笔多少支．
<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>