The answer is 2:1.
Steps:
1. You do 750 divide by 3, which is 250
2. Now you know that 250 is number of kids who do ride the bus
3. You do 750-250 which equals 500
4. 500 is the kids who don't ride the bus
5. You do 500:250 then you divide each side by 250 to simplify your equation
6. Your answer is 2:1