Ding Ding and Nini worked together to type a 3,200-word document, and it took 20 minutes to finish typing. Ding Ding's speed is 90 words per minute, so how many words per minute is Nini's speed?
丁丁和妮妮合作打一份3200字的文件，打了20分钟刚好打完，丁丁的速度是每分钟90字，那么妮妮的速度是每分钟多少字？
<gadget id="calculator">3_200 / 20</gadget>
<output>160</output>

<gadget id="calculator">160 - 90</gadget>
<output>70</output>

<result>70</result>