$4000 is invested in two different accounts. One account pays 6% simple interest per year and the other account pays 18% simple interest per year. If the total interest earn in one year is $432.00, how much is invested at each rate?

Let $LaTeX: \displaystyle x$ be the amount invested at 6%. Then $LaTeX: \displaystyle 4000-x$ is amount invested at 18%. This gives the equation $LaTeX: \displaystyle 0.06 x+0.18(4000 - x)=432$ . Solving for $LaTeX: \displaystyle x$ gives $LaTeX: \displaystyle x = 2400$ . So the amount in the other account is $1600.