Example

Solving a Punch Mixture Problem Using a System of Equations

Apply the seven-step problem-solving strategy for systems of linear equations to a real-world mixture problem solved by graphing.

Problem: Sondra is making 10 quarts of punch from fruit juice and club soda. The number of quarts of fruit juice is 4 times the number of quarts of club soda. How many quarts of fruit juice and how many quarts of club soda does Sondra need?

  1. Read the problem.
  2. Identify what to find: the number of quarts of fruit juice and the number of quarts of club soda.
  3. Name the unknowns: Let ff = number of quarts of fruit juice. Let cc = number of quarts of club soda.
  4. Translate into a system of equations. The total amount of punch is 10 quarts: f+c=10f + c = 10. The fruit juice is 4 times the club soda: f=4cf = 4c. The system is:

{f+c=10f=4c\left\{\begin{array}{l} f + c = 10 \\ f = 4c \end{array}\right.

  1. Solve by graphing both equations on the same coordinate system. The line f=4cf = 4c has slope 44 and y-intercept 00. The line f+c=10f + c = 10 can be rewritten as f=c+10f = -c + 10, with slope 1-1 and y-intercept 1010. The two lines intersect at the point (2,8)(2, 8).

  2. Check: Is 8 quarts of fruit juice 4 times 2 quarts of club soda? 4×2=84 \times 2 = 8 ✓. Do 8 quarts of fruit juice plus 2 quarts of club soda equal 10 quarts of punch? 8+2=108 + 2 = 10 ✓.

  3. Answer: Sondra needs 8 quarts of fruit juice and 2 quarts of club soda.

This example demonstrates how a real-world mixture scenario naturally produces two equations — one from the total-quantity constraint and one from a multiplicative relationship between the two ingredients — which together form a system that can be solved by graphing.

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Updated 2026-04-21

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