Such as 16: 1 (corresponding to 16), 2 (corresponding to 8), 4 (corresponding to itself), 8, 16- 16, and the switching times are 5.
While others are divided into even factors:
Such as 18: 1 (corresponding to 18), 2 (corresponding to 9), 3 (corresponding to 6), 6, 9, 18- 18, and the switching times are 6.
The number divided by the divisor is the number of switches. When the number of switches is odd, the light goes out.
LZ: You can try No.6 lamp, 12 lamp and 18 lamp. These are all multiples of 6, but they are all on.