An express train and an ordinary passenger train depart from cities A and B at the same time. The express train travels 90 kilometers per hour, and the ordinary passenger train travels 48 kilometers per hour. After 4 hours, the two trains meet on the way. Find the length of the railway between the two cities A and B?
一列快车和一列普通客车从甲乙两个城市同时相对开出，快车每小时行90千米，普通客车每小时行48千米，经过4小时后，两列火车在途中相遇．求甲乙两城市间的铁路长多少千米？
<gadget id="calculator">90 + 48</gadget>
<output>138</output>

<gadget id="calculator">138 * 4</gadget>
<output>552</output>

<result>552</result>