person goes to a store to buy two kinds of pens, red and blue. The price of the red pen is 5 yuan, and the price of the blue pen is 9 yuan. Due to the large amount of purchases, the store gives a discount: 15% off for the red pen and 20% off for the blue pen. It turned out to save 18%. It is known that he bought 30 blue pens, so how many red pens did he buy?
某人到商店买红蓝两种笔，红笔定价5元，蓝笔定价9元，由于购买量较多，商店给予优惠：红笔8.5折，蓝笔8折，结果此人付的钱比原来节省了18%，已知他买了蓝笔30支，那么红笔买了多少支．
<gadget id="calculator">30 * 9</gadget>
<output>270</output>

<gadget id="calculator">18 / 100</gadget>
<output>9/50 = around 0.18</output>

<gadget id="calculator">30 * 9 * (9/50)</gadget>
<output>243/5 = around 48.6</output>

<gadget id="calculator">30 * 9 * 0.8</gadget>
<output>216</output>

<gadget id="calculator">270 - (243/5) - 216</gadget>
<output>27/5 = around 5.4</output>

<gadget id="calculator">5 * (9/50)</gadget>
<output>9/10 = around 0.9</output>

<gadget id="calculator">5 * 0.85</gadget>
<output>4.25</output>

<gadget id="calculator">4.25 - 5</gadget>
<output>-0.75</output>

<gadget id="calculator">(9/10) + (-0.75)</gadget>
<output>0.15</output>

<gadget id="calculator">(27/5) / 0.15</gadget>
<output>36</output>

<result>36</result>