The Bureau of Statistics has counted 664 cities, which can be divided into three categories according to the air pollution: good, lightly polluted and seriously polluted. Among them, the number of cities with good air quality is 52 times more than the number of heavily polluted cities, and the number of lightly polluted cities is twice the number of heavily polluted cities. How many cities are there with severe air pollution?
统计局统计了664座城市，按空气污染情况可分为三类：良好，轻度污染和严重污染．其中，空气质量良好的城市数比严重污染城市数的3倍多52座，轻度污染城市数是严重污染城市数的2倍．则空气严重污染城市有多少座．
<gadget id="calculator">664 - 52</gadget>
<output>612</output>

<gadget id="calculator">3 + 2 + 1</gadget>
<output>6</output>

<gadget id="calculator">612 / 6</gadget>
<output>102</output>

<result>102</result>