Help with stats!! Quick Start Company makes 12-volt car batteries. After many years of product testing, the company knows that the average life of a Quick Start battery is normally distributed, with a mean of 46.6 months and a standard deviation of 6.9 months.
(a) If Quick Start guarantees a full refund on any battery that fails within the 36-month period after purchase, what percentage of its batteries will the company expect to replace? (Round your answer to two decimal places.)

Question

Help with stats!! Quick Start Company makes 12-volt car batteries. After many years of product testing, the company knows that the average life of a Quick Start battery is normally distributed, with a mean of 46.6 months and a standard deviation of 6.9 months.
(a) If Quick Start guarantees a full refund on any battery that fails within the 36-month period after purchase, what percentage of its batteries will the company expect to replace? (Round your answer to two decimal places.)

anonymous · Answer

36 months is 1.53 standard deviations below the mean.
Calculating the percentage based on a z-score of -1.53 gives 6.43%

anonymous · Answer

how did you know that 36 months is 1.53 standard deviations below the mean?

anonymous · Answer

The mean is 46.6 months,
the standard deviation is 6.9 months.

46.6 months - 36 months = 10.6 months

10.6 months / 6.9 months = 1.536 months.

anonymous · Answer

Oooh got it. But then how did you get the percentage from the zscore? Did you use the "Ncdf" function on the calculator?