Xiaoli went to the bookstore from home. She walked for 0.5 hours first, and then took the bus for 0.75 hours, covering a total of 30 kilometers. If she walks at 6 kilometers per hour, how many kilometers per hour does the bus travel on average?
小丽从家去书店，她先步行走了0.5小时，又坐公交车0.75小时，共行了30千米．如果她步行速度是每小时6千米，那么公交车平均每小时行驶多少千米？
<gadget id="calculator">6 * 0.5</gadget>
<output>3</output>

<gadget id="calculator">30 - 3</gadget>
<output>27</output>

<gadget id="calculator">27 / 0.75</gadget>
<output>36</output>

<result>36</result>