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Example

Solving a Proportion Application: Currency Exchange from Dollars to Pesos

Apply the proportion-solving strategy to a currency exchange problem.

Problem: Josiah went to Mexico for spring break and changed $325 into Mexican pesos. At that time, the exchange rate was $1 US equal to 12.54 Mexican pesos. How many Mexican pesos did he get for his trip?

  1. Identify what to find and assign a variable: How many Mexican pesos did Josiah get? Let pp = the number of Mexican pesos.
  2. Translate into a proportion, keeping units consistent — dollars over pesos equals dollars over pesos:

112.54=325p\frac{1}{12.54} = \frac{325}{p}

  1. Solve by multiplying both sides by the LCD, 12.54p12.54p:

12.54p112.54=12.54p325p12.54p \cdot \frac{1}{12.54} = 12.54p \cdot \frac{325}{p}

Remove common factors on each side. On the left, 12.5412.54 cancels, leaving pp. On the right, pp cancels, leaving 12.5432512.54 \cdot 325:

p=4075.5p = 4075.5

  1. Check reasonableness: $100 would be 1,254 pesos. Since $325 is a little more than 3 times $100, the answer of 4,075.5 pesos makes sense (roughly 3×1,254=3,7623 \times 1{,}254 = 3{,}762, and 4,075.5 is close to that estimate).
  2. Verify by substituting p=4075.5p = 4075.5 back into the original proportion:

112.54=3254075.5\frac{1}{12.54} = \frac{325}{4075.5}

0.079740.079740.07974 \approx 0.07974

Answer: Josiah got 4,075.5 Mexican pesos for his spring break trip.

This example demonstrates that proportion problems involving currency exchange follow the same procedure as other proportion applications: align matching units on each side of the proportion (dollars over pesos = dollars over pesos), multiply both sides by the LCD to clear the fractions, and verify the result. The number of pesos is proportional to the number of dollars — when two quantities are related by a proportion, they are said to be proportional.

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Updated 2026-04-21

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