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.