Random subtraction question generator for two numbers

An excel document that will generate subtraction questions for two numbers with the option to exclude sums with negative answers. The numbers to subtract will be randomly generated between an upper and lower limit so its easy to differentiate by changing the range. The answer will display when button is clicked and a countdown timer will sound after the time selected has elapsed. A fun activity for children to do individually on laptops or as a group on an interactive whiteboard.