t-tests for multiple comparisons, the Bonferonni correction

This is a continuation of the previous problem.

Here's the Spring 2014 survey responses of 922 Stat 100 students to the survey question: "20 years down the road, what do you expect your income to be?" Are there differences between the 5 ethnic groups (White, Black, Hispanic, Asian, Mixed/Other) on how much they expect to be earning 20 years from now? Here are the rounded results.

Assume the 922 students were randomly chosen from all 31,000 U of I undergrads.

1) How many degrees of freedom are there?