In a recipe that uses 4 parts flour and 5 parts sugar, if the total parts are 45, how many parts are flour?

• A. 25
• B. 20
• C. 18
• D. 40

Answer: (B)

Think of the recipe as a whole made of equal “parts.” Flour and sugar share a fixed ratio of 4 parts flour to 5 parts sugar, which adds up to 9 parts total. Flour makes up 4 of those 9 parts, so flour is 4/9 of the entire mixture. If the total is 45 parts, flour equals 45 × 4/9 = (45/9) × 4 = 5 × 4 = 20 parts. You can check by splitting 45 into nine equal groups of 5 parts each; four of those groups give 20 parts for flour and the remaining five groups give 25 parts for sugar, keeping the 4:5 ratio. Other options would correspond to different shares of the total (for example, 25 parts would be 5/9 of the total, not 4/9), so 20 is the correct amount for flour.