Yumiko invests $2100 in a Treasury Bond at 8.9% interest compounded 4 times per year.

  1. Find the equation that represents the account balance at time LaTeX:  \displaystyle t .
  2. When will the account double? e.g. when will it be $4200. Round your solution to the nearest tenth of a year.

  1. Using the compounded interest formula gives LaTeX:  \displaystyle A = 2100 \cdot 1.02225^{4 t} .
  2. It will take about 7.9 years.