A major retail clothing store is interested in estimating the difference in mean monthly purchases by customers who use the store's in-house credit card versus using a Visa,Mastercard,or one of the other major credit cards.To do this,it has randomly selected a sample of customers who have made one or more purchases with each of the types of credit cards.The following represents the results of the sampling: 
Suppose that the managers wished to test whether there is a statistical difference in the mean monthly purchases by customers using the two types of credit cards,using a significance level of .05,what is the value of the test statistic assuming the standard deviations are known?
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