What percent of the squares are not covered by checker pieces at the beginning of a game of checkers?

It depends. There are 24 checkers and 64 squares on the board, including the red squares. 24/64 = 37.5%. 100 - 37.5 = 72.5%. However, if you mean just the black squares there are 32 available squares. 24/32 = 75%. 100 - 75 = 25%