Avery invests $2000 in saving certificates at an interest rate of 12% per year compounded quarterly. How long will it take for this sum to double?
A = P(1 + i)^n
4000 = 2000((1 + 0.12) / 4)^4n
2 = 0.28^4n
log2 = 4nlog0.28
log2 / log0.28 = 4n
-0.5445 = 4n
-0.13612 = n
Now a year can't be negative so something fishy is going on here.