Xiao Ming walks 90 meters per minute, and walks 4 minutes from home to the library. How many meters does it take to go back and forth?
小明每分钟走90米，从家到图书馆走了4分．往返一次需走多少米？
<gadget id="calculator">90 * 4 * 2</gadget>
<output>720</output>

<result>720</result>