A bank wonders whether omitting the annual credit card fee for customers who charge at least $2400 in a year would increase the amount they charge on its credit card. The bank makes this offer to a...

A bank wonders whether omitting the annual credit card fee for customers who charge at least $2400 in a year would increase the amount they charge on its credit card. The bank makes this offer to a random sample of 15 of its existing credit card customers who charge at least $2400 a year. It then compares how much these customers charge this year with the amount that they charged last year. The sample mean increase is $730, and the sample standard deviation is $140. Construct a 95% confidence interval for the average increase in spending .