Xiao Zhang has 200 pencils, Xiao Li has 40 pens, each time Xiao Zhang gives Xiao Li 6 pencils, Xiao Li returns Xiao Zhang 1 pen, after how many times of such exchanges, the number of pencils in Xiao Zhang’s hand is The number of pens in Xiao Li's hand is 2 times.
小张有200支铅笔，小李有40支钢笔，每次小张给小李6支铅笔，小李还给小张1支钢笔，经过多少次这样的交换后，小张手中铅笔的数量是小李手中钢笔数量的2倍．
<gadget id="calculator">40 * 2</gadget>
<output>80</output>

<gadget id="calculator">200 - 80</gadget>
<output>120</output>

<gadget id="calculator">6 - 2</gadget>
<output>4</output>

<gadget id="calculator">120 / 4</gadget>
<output>30</output>

<result>30</result>