It takes Dongdong 5 minutes to walk from home to school, with an average of 90 meters per minute. How many meters does he need to walk back and forth?
东东从家里到学校要走5分钟，平均每分钟走90米，他来回一趟要走多少米．
<gadget id="calculator">90 * 5 * 2</gadget>
<output>900</output>

<result>900</result>