Example

Solving a Dual Student Loan Interest Application Using a System of Equations

Apply the seven-step problem-solving strategy and a mixture table to find the principal given the amount of interest earned. Problem: Laura owes $18{,}000 on her student loans. The interest rate on the bank loan is 2.5% and the interest rate on the federal loan is 6.9%. The total amount of interest she paid last year was $1{,}066. What was the principal for each loan? 1. Read the problem. 2. Identify what to find: the principal of each loan. 3. Name the unknowns. Let bb = the principal for the bank loan, and ff = the principal for the federal loan. Organize into a table: / Loan / Principal ($) / Rate / Interest ($) / /---/---/---/---/ / Bank / bb / 0.025 / 0.025b / / Federal / ff / 0.069 / 0.069f / / Total / 18{,}000 / / 1{,}066 / 4. Translate into a system of equations: \left\{\begin{array}{l} b + f = 18{,}000 0.025b + 0.069f = 1{,}066 \end{array} ight. 5. Solve the system using substitution. Solve the first equation for bb: b=f+18,000b = -f + 18{,}000 Substitute into the second equation: 0.025(f+18,000)+0.069f=1,0660.025(-f + 18{,}000) + 0.069f = 1{,}066 0.025f+450+0.069f=1,066-0.025f + 450 + 0.069f = 1{,}066 0.044f+450=1,0660.044f + 450 = 1{,}066 0.044f=6160.044f = 616 f=14,000f = 14{,}000 To find bb, substitute f=14,000f = 14{,}000 into the first equation: b+14,000=18,000b + 14{,}000 = 18{,}000 b=4,000b = 4{,}000 6. Check the answer. 4,000+14,000=18,0004{,}000 + 14{,}000 = 18{,}000 checkmark 0.025(4{,}000) + 0.069(14{,}000) = 100 + 966 = 1{,}066 checkmark 7. Answer the question. The principal for the bank loan was $4{,}000 and the principal for the federal loan was $14{,}000.

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Updated 2026-06-05

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