5% / 1200 =0.004166667 - monthly interest rate.
2,000 /1,000 =1.004166667^n, where n = number of months to double the money
2 = 1.004166667^n take the log of both sides
n =Log(2) / Log(1.004166667)
n =166.7 months to double the money.Or:166 months - the least integer number of months.
n = 167 months when Kimberly will owe: $2,002.48
Note: I have taken your interest rate to mean "5% annual rate compounded monthly." Your question says "5% compounded monthly", which is equivalent to 79.59% effective annual rate.
If you DO MEAN 5% compounded monthly, then the above calculation will look like this:
2,000/1,000 =1.05^n
2 = 1.05^n
n = Log(2) / Log(1.05)
n =14.21 months, or: 14 - the least integer number of months, after which:
n = 15 months when Kimberly will owe $2,078.93