A mixed number is equal to sum a integer plus a fraction less than 1. The result is a fraction where the numerator will be always greater than the denominator (This fraction is called an improper fraction)
If the goal is to sell at least $1500 worth of tickets then what you can do is add the 12.50+ 6.25 which well equal 18.75. Then you can divide 1500/18.75 which eqauls to 80 meaning at least 80 seats must be sold