There are two ships, A and B, and the ship A and the drifting object are traveling from the west of the river to the east at the same time, and the ship B is also traveling from the east to the west at the same time. Ship A travels for 4 hours and is 100 kilometers away from the drifting object. Ship B encounters the drifting object after traveling for 12 hours. The speed of the two boats is the same, how many kilometers is the length of the river?
有甲、乙两船，甲船和漂流物同时由河西向东而行，乙船也同时从东向西而行．甲船行4小时后与漂流物相距100千米，乙船行驶12小时后，与漂流物相遇．两船的船速相同，河长多少千米？
<gadget id="calculator">100 / 4</gadget>
<output>25</output>

<gadget id="calculator">25 * 12</gadget>
<output>300</output>

<result>300</result>