Typist A typed an average of 100 characters per minute, typist B typed an average of 60 characters per minute, and a manuscript was 600 characters. After A had typed alone for several minutes, he left due to business, and B continued to type. minutes, how many minutes did Typist A type?
甲打字员平均每分钟打100个字，乙打字员平均每分钟打60个字，一份稿件600个字，甲单独打了若干分钟后，因事离开，由乙接着打完，共用了7分钟，甲打字员打了多少分钟？
<gadget id="calculator">7 * 60</gadget>
<output>420</output>

<gadget id="calculator">600 - 420</gadget>
<output>180</output>

<gadget id="calculator">100 - 60</gadget>
<output>40</output>

<gadget id="calculator">180 / 40</gadget>
<output>9/2 = around 4.5</output>

<result>9/2 = around 4.5</result>