A random sample of 160 car purchases are selected and categorized by

A random sample of 160 car purchases are selected and categorized by age. The results are listed below. The age distribution of drivers for the given categories is 18% for the under 26 group, 39% for the group, 31% for the 45-65 group, and 12% for the group over 65. Find the critical value to test the claim that all ages have purchase rates proportional to their driving rates. 

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