A study by a bank compared the average savings of customers who were depositors for three years or less, with those who had been depositors for more than three years. The results of a sample are:

less than equal too 3 years.

Mean savings balance $1200

population standard deviation $ 100

Sample size 100

greater than 3 years

mean savings balance $1,250

population standard deviation $250

sample size 150

to test the alternate hypothesis that the two groups of customers have different savings rates, what is the critical value of z using a significance level of 0.05?