Donna De Paul is raising money for the homeless. She discovers that each church group requires 2 hours of letter writing and 1 hour of follow-up, while for each labor union she needs 2 hours of letter writing and 3 hours of follow-up. Donna can raise $125 from each church group and $175 from each union local, and she has a maximum of 12 hours of letter writing and a maximum of 16 hours of follow-up available per month. Determine the most profitable mixture of groups she should contact and the most money she can raise in a month.

Let x1 be the number of church groups, and let x2 be the number of labor unions. What is the objective function?