Well 1/3 for savings means that you save 1/3*4500 or 1500 dollars. You then have $3000 left. Of this if you spend 50% on food, you spend 0.5*3000 or 1500 dollars on food.
5/20 can also be represented as 25/100 or 25%. The can be done by multiplying the top and bottom both by 5 and then dividing our numerator (25) by our denominator(100) to get the %.
