normal probability distribution

Question

If I as just installed 1400 new lightbulbs with an expected mean lifespan of 60 months and a lifespan standard deviation of 10 months. How many bulbs will need to be replaced after 44 months?

I work this out on excel normsdist(44,60,10,true) but this was wrong I think I need to do something with the 1400 but not sure. thank you

Answer 1

Probability of a bulb that it will work after 44 months:

normsdist(44,60,10,true)

How many of 1400 bulbs

1400*normsdist(44,60,10,true)

Answer 2

The probability that an individual bulb needs replacing: $$P (Z<-1,6) = 0.0548$$ For Your set of data: $$0.0548\cdot 1400 = 76,72 $$