The fast and slow trains drive separately from the two places and meet each other in 4 hours. The express train takes 7 hours to complete the journey. The slow train travels 51 kilometers per hour. How many kilometers per hour does the express train travel?
快慢两车分别从两地对开，4小时相遇，快车行完全程要7小时，慢车每小时行51千米，快车每小时行多少千米．
<gadget id="calculator">51 * 4</gadget>
<output>204</output>

<gadget id="calculator">7 - 4</gadget>
<output>3</output>

<gadget id="calculator">204 / 3</gadget>
<output>68</output>

<result>68</result>