15. In the amusement hall of the Youth Palace, there are 250 colored light bulbs, numbered from 1 to 250. Their on-off rules are: at the 1st second, all light bulbs turn on; at the 2nd second, all light bulbs with numbers that are multiples of 2 turn off; at the 3rd second, all light bulbs with numbers that are multiples of 3 change their state, i.e., on bulbs turn off and off bulbs turn on; at the nth second, all light bulbs with numbers that are multiples of n change their state. This continues, and at the 250th second, the number of light bulbs that are on is ( 

将上面的文本翻译成英文，请保留源文本的换行和格式，直接输出翻译结果。
15
【Answer】According to the problem, the number of times a light changes from off to on (or vice versa) is equal to the number of divisors of that number; and initially all the bulbs are off; therefore, if a number has an odd number of divisors, then the bulb corresponding to this number will be on at the 250th second; and only perfect squares have an odd number of divisors; hence, this problem is asking for the number of perfect squares between $1 \sim 250$; so the answer is 15.