Equal amounts are invested at 2%, 7%, and 9% annual interest. If the three investments yield a total of $792 annual interest, find the total investment.

Answer:[tex]\large \boxed{\$13 200}[/tex]Step-by-step explanation:The formula for simple interest I is I = Prt, where P = the principal (amount invested) r = the interest rate per period and n = the number of periods In this problem, the interest rate is annual, so n = 1. We must write the interest rate as a decimal fraction. Let x = amount of each investment. Then 0.02x = interest at 2 % 0.07x = interest at 7 % 0.09x = interest at 9 % 0.18x  = total interest [tex]\begin{array}{rcl}0.18x & = & 792\\x & = & \dfrac{792}{0.18}\\& = & 4400\\\end{array}\\\text{Each investment was \$4400.}\\\text{The total investment was 3$\times \$4400$ or $\large \boxed{\mathbf{\$13 200}}$}[/tex]