Charles is planning to have a party for Laura's 16th birthday. He will be making wooden benches and tables. He estimates that a wooden bench will take 4 hours to complete and a wooden table will take 2 hours; he plans to work no more than 60 hours per week. Charles wants to make at least 4 tables and at least 8 benches. If the wood costs $40 for a table and $25 for a bench, find a combination of benches and tables that will minimize his cost so that he has more money for Laura's birthday present.