Question

A local bank has determined that the daily balances of the checking accounts of its customers are normally distributed with an average of $280 and a standard deviation of $20.

a. What percentage of its customers has daily balances of more than $275?
b. What percentage of its customers has daily balances less than $243?
c. What percentage of its customers' balances is between $241 and $301.60?

Answer 1

Explanation:

Need a z-score with all of these, which essentially makes the table needed a standard normal table with mean 0 and sd 1

z=(x-mean)/sd

a. (275-280)/20=-0.25

want the probability of z <-0.25, and that is 0.4013 or 40.13%

b.(243-280)/20=-1.85, and probability of z < -1.85 is 0.0322 or 3.22%

c. this is z of -39/20 or -1.95 and z of 21.60/20 or 1.08. This has a probability of 0.8343 or 83.43%

Use 2nd VARS 2 for normal cdf and put in the numbers, with at least -6 or +6 for minus or plus infinity. 1E99 is fine but isn't really necessary.