Question

10000 ‘Roshani’ light bulbs with a mean life of 150 days are installed in a new factory and their length of life is normally distributed with standard deviation of 10 days. How many Roshani-bulbs will expire in less than 160 days? 

Answer 1

$P(X<160)=P(Z<\frac{160-150}{10})=P(Z<1)=0.8413.$

Number of Roshani-bulbs which will expire in less than 160 days:

$n=10000*0.8413=8413.$