I am really having trouble breaking this problem down into a formula. It just doesn't feel like there is enough information, but I know that there is. My instructor provides no examples and the book has none on this one specifically. I am hoping someone can help me solve it, or atleast get it started. Any help much appreciated.

A company invests a total of $30,000 of surplus funds at two annual rates of interest: 5% and 6.75%. It wishes an annual yield of no less than 6.5%. What is the least amount of money that the company must invest at the 6.75% rate.