A manuscript is 10,000 words. A typed 120 words per minute, and B typed 80 words per minute. After A alone typed for a few minutes, B continued to type for some reason. The two shared 90 minutes. How many minutes did A spend typing?
一份稿件1万字，甲每分钟打120字，乙每分钟打80字，现甲单独打字打若干分钟后，因有事由乙接着打，两人共用了90分钟，甲打字用了多少分钟？
<gadget id="calculator">90 * 80</gadget>
<output>7_200</output>

<gadget id="calculator">10_000 - 7_200</gadget>
<output>2_800</output>

<gadget id="calculator">120 - 80</gadget>
<output>40</output>

<gadget id="calculator">2_800 / 40</gadget>
<output>70</output>

<result>70</result>