Suppose researchers are interested in whether consumers spend more on a trip to store A or store B and collected a systematic sample from 80 store A customers and 95 store B customers by asking customers for their purchase amounts as they left the stores. The data collected are summarized in the table below. The researchers want the margin of error for the estimate of the mean difference between the twostores' purchase amounts to be $5.50, with95% confidence. How big should the sample size be(assuming two equal samplesizes)? Use the standard deviations given as your estimates.

Store A

n = 80

y = 44

s = 25

Store B

n = 95

y = 57

s = 15

Sample size should be at least?