A battery has an average life span of 50 hours, with a standard deviation of 3 hours. The life span of the batteries is normally distributed. What percent of the batteries last at least 44 hours?

Question

mrnood · Answer

how many standard deviations below the mean is 44hours (mean is 50?)

mathmale · Answer

Mr. Nood's question is appropriate.  Stated just a lilttle differently:  Find the z-score corresonding to a battery life of 44 hours, given that the mean is 50 and the std. dev. is 3.

mathmale · Answer

Hint:  because 44 is below the mean, the z score in question will be negative.

mrnood · Answer

then use
68–95–99.7 rule

to work out what proportion last LESS than 44 hours 
the rest last MORE than 44 hours

mathmale · Answer

@ciaradrake:  would you please type in the formula for "z-score."

mathmale · Answer

Alternatively, using the "68-95-99.7 rule" would enable you to bypass the z-score business entirely.

mrnood · Answer

gone offline I think!

mathmale · Answer

;(