We are doubling the initial investment so y = 2 y0
2 y0 = y0 e ^( .0325t)
Divide each side by y0
2 = e ^( .0325t)
Take the natural log of each side
ln(2) = ln( e ^( .0325t))
ln2 = .0325t
Divide each side by .0325
ln(2) / .0325 = t
21.32760 = t
Rounding to the nearest tenth
21.3 = t