A proportion is two equivalent fractions. One of the most common ways to write a proportion for a real-life situation is:

Part 1 Whole 1 = Part 2 Whole 2

Example: A car was purchased for $5000. The car payments are $200 per month. Another car is purchased for $8000. How much will the monthly payment be for second car?

The situation written as a proportion is:

Car payment 1 Car value 1 = Car payment 2 Car value 2

OR

200 5000 = x 8000

In order to solve for the missing car payment, cross-products are used.

Cross products are the product of the numerator of the first ratio (200) and the denominator of the second ratio (8000) equals the product of the denominator of the first ratio (5000) and the numerator of the second (x).

The monthly payment for the second car is $320.

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