Example

Solving an Investment Interest Application Using a System of Equations

To determine the amount of money to invest in different funds to achieve a target interest, we can model the application with a system of equations using the simple interest formula I=PrtI = Prt. For example, if a total principal of $ 40,00040{,}000 is invested across a stock fund ss earning 8%8\% and bonds bb earning 3%3\%, to earn an overall interest of 7.1%7.1\%, the system is derived from two columns:

  1. The Principal equation represents the total amount invested: s+b=40,000s + b = 40{,}000.
  2. The Interest equation represents the interest earned: 0.08s+0.03b=0.071(40,000)0.08s + 0.03b = 0.071(40{,}000). Solving this system by elimination or substitution yields the principal for each investment.

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Updated 2026-05-26

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