Example

Solving a Coin Mixture Problem Using a System of Equations

Apply the seven-step problem-solving strategy and a table to solve a coin mixture application using a system of linear equations.

Problem: Juan has a pocketful of nickels and dimes. The total value of the coins is $8.10. The number of dimes is 99 less than twice the number of nickels. How many nickels and how many dimes does Juan have?

  1. Read the problem. A table will help organize the information.
  2. Identify what to find: the number of nickels and the number of dimes.
  3. Name the unknowns. Let n=n = the number of nickels and d=d = the number of dimes. Organize the data into a table by listing the coin types, their quantities, their monetary values, and their total values:
TypeNumberValue ($)Total Value ($)
Nickelsnn0.050.050.05n0.05n
Dimesdd0.100.100.10d0.10d
Total8.108.10
  1. Translate into a system of equations.

    • The "Total Value" column gives the first equation: 0.05n+0.10d=8.100.05n + 0.10d = 8.10
    • The relationship between the quantities (the number of dimes is 99 less than twice the number of nickels) gives the second equation: d=2n9d = 2n - 9

    The system of equations is: {0.05n+0.10d=8.10d=2n9\left\{\begin{array}{l} 0.05n + 0.10d = 8.10 \\ d = 2n - 9 \end{array}\right.

  2. Solve the system using the substitution method. Substitute 2n92n - 9 for dd in the first equation: 0.05n+0.10(2n9)=8.100.05n + 0.10(2n - 9) = 8.10

    Distribute the 0.100.10: 0.05n+0.20n0.90=8.100.05n + 0.20n - 0.90 = 8.10

    Combine like terms: 0.25n0.90=8.100.25n - 0.90 = 8.10

    Add 0.900.90 to both sides: 0.25n=9.000.25n = 9.00

    Divide by 0.250.25: n=36n = 36

    Substitute n=36n = 36 into the second equation to find dd: d=2(36)9=729=63d = 2(36) - 9 = 72 - 9 = 63

  3. Check the result. 3636 nickels at $0.05 each is $1.80. 6363 dimes at $0.10 each is $6.30. 1.80+6.30=8.101.80 + 6.30 = 8.10 \checkmark

  4. Answer the question. Juan has 3636 nickels and 6363 dimes.

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Updated 2026-05-25

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Intermediate Algebra @ OpenStax

Ch.4 Systems of Linear Equations - Intermediate Algebra @ OpenStax

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