500
How many balloons of each color would you need to add to the original bag to make the probability of drawing a red balloon 1/2 ?
Some possibilities: Add 1 yellow and 4 oranges, or 2yellows and 3 oranges, or 3 yellows and 2 oranges, or 4 yellows and 1 orange, or 5 yellows, or 5 oranges.