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How to Calculate Ratios and Proportions in Math

Contributor
By James McIlhargey
eHow Contributing Writer
(20 Ratings)

Ratios and Proportions are a very important part of math for those just starting off. They describe the relationship between two amounts by a single number. They are used everywhere in life; how many eggs to cups flour in the cookie recipe and how many miles per gallon does a car consume are two examples.

Difficulty: Easy
Instructions

Things You'll Need:

  • Paper
  • Pencil or Pen

    How to compute ratios and proportions

  1. Step 1

    Identify the amounts that are being compared. If comparing apples to oranges, then the amount of apples and the amount of oranges must be known. Express each as a number. For example, let's say I have 20 apples and 10 oranges.

  2. Step 2

    Decide which will be the first quantity. Are apples going to be described first, or oranges?

  3. Step 3

    Divide the first quantity by the second quantity. If I choose apples, then I would divide as follows:number of apples (20) divided by (/) number of oranges (10) equals (=) proportion of apples to oranges (2)20/10 = 2

  4. Step 4

    Describe the result in words. From the result of step 3, I would have 2 apples for every orange.

  5. Other examples

  6. Step 1

    Lets say your car used 10 gallons to go 300 miles. Compute the miles per gallon for your car.

  7. Step 2

    Amount of miles (300) divided by amount of gas (10) equals miles per gallon300 / 10 = 30 miles per gallon.

  8. Step 3

    You and your four friends won 700 dollars. If the five of you get equal shares, how much money does each person get?

  9. Step 4

    Amount of money (700) divided by number of people (5) is the money per person.700 / 5 = 140 dollars per person.

  10. Step 5

    If a batch of cookies needs 3 eggs, how many batches can you cook with 12 eggs?

  11. Step 6

    In this problem, we use the ratio given.12 eggs divided by 3 eggs per batch = 4 batches.

Tips & Warnings
  • Always try to express the problem as an A per B
  • In real life, the numbers are not always simple. Use a calculator when needed.

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