Someone goes from A to B. If he rides a motorcycle for 12 hours from place A, and then changes to ride a bicycle for 9 hours, he just arrives at place B. If he rides a bicycle for 21 hours from place A, and then changes to ride a motorcycle for 8 hours, he will arrive at place B just in time. Q: How many hours does it take to get to B by motorcycle?
某人由甲地去乙地．如果他从甲地先骑摩托车行12小时，再换骑自行车行9小时，恰好到达乙地．如果他从甲地先骑自行车行21小时，再换骑摩托车行8小时，也恰好到达乙地．问：全程骑摩托车需要几小时到达乙地？
<gadget id="calculator">12 - 8</gadget>
<output>4</output>

<gadget id="calculator">21 - 9</gadget>
<output>12</output>

<gadget id="calculator">4 / 12</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">9 * (1/3)</gadget>
<output>3</output>

<gadget id="calculator">12 + 3</gadget>
<output>15</output>

<result>15</result>