Teacher Yu took a car when he went to work, and walked when he returned home. It took a total of 90 minutes on the road; How many minutes will it take to walk both ways.
于老师上班时坐车，回家时步行，在路上一共花90分钟；往返都坐车，只需30分钟．如果往返都步行，需要多少分钟．
<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">90 - 15</gadget>
<output>75</output>

<gadget id="calculator">75 * 2</gadget>
<output>150</output>

<result>150</result>