classmate walks to school and takes the bus after school. It takes a total of 90 minutes on the road. If he takes the bus both times, it only takes 30 minutes. How many minutes does it take if he walks both ways?
某同学上学时步行，放学时乘坐公交车，路上一共要用90分钟，若是往返都乘坐公交车，只需要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>