Across the country, families of two adults and two children currently have an average weekly shopping bill at a large supermarket chain of $255. However, you believe the weekly bill for this type of family in your local area is more than the national average figure. You decide to test your claim by randomly selecting 21 families of two adults and two children who shop at this supermarket chain in your local area. The data collected are presented in the table below. Assuming the bill amounts for families shopping at this supermarket chain are normally distributed and using a 5% level of significance, is there evidence to support your belief?

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212 270 320 220 288 323 225 289 241 249 299 255 304 262 315 234 295 331 296 334 328