I'm not that great when it comes to Russian, but hope you understant the solution in English.
For each lightbulb has 2 states (on & off), which means that each added lightbulb offers twice as much (2x) signals.
x lightbulbs = 2^x signals.
8 < 15 < 16, which is the same as 2^3 < 15 < 2^4. So your answer is 4.
