There are 250 colored light bulbs hanging in the juvenile official amusement hall, numbered according to 1-250. Their light and dark rules are: in the first second, all the bulbs become brighter; in the second second, all the bulbs numbered in multiples of 2 change from bright to dark; in the third second, all the bulbs numbered in multiples of 3 change their original brightness State, that is, bright dimming, dark brightening; in the nth second, all bulbs numbered as multiples of n change their original bright and dark states. If this continues, how many light bulbs are on at the 250th second?
少年官游乐厅内悬挂着250个彩色灯泡，按1-250编号．它们的亮暗规则是：第1秒，全部灯泡变亮；第2秒，凡是编号为2的倍数的灯泡由亮变暗；第3秒，凡是编号为3的倍数的灯泡改变原来的亮暗状态，即亮的变暗，暗的变亮；第n秒，凡编号为n的倍数的灯泡改变原来的亮暗状态．这样继续下去，第250秒时，亮着的灯泡有多少个．
<result>15</result>