Let 
be a probability that i-th light fails, where i=1,2,3. Then, 
is the probability that i-th light remain bright.
<span>If the lights fail independently of each other, the probability that a string of lights will remain bright for two years is:
</span>
.
<span>If the lights fail independently of each other, the probability that a string of lights will remain bright for two years is:
</span>