500
25 balloons: 15 red, 3 yellow, 7 orange - How many ballons 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, 2 yellow and 3 oranges, 3 yellow and 2 oranges, 4 yellows and 1 oranges, 5 yellows, or 5 oranges