The time that Carlene would have to wait to buy the desk is 4 years.
When an amount of money is compounded, it means that both the amount that was invested and the interest accrued earn interest.
The formula that can be used to determine the time is
N = log(FV / PV) / log(e) / interest rate
log(500 / 400) / log(e) / 0.06 = 3.71 years = 4 years
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