Puzzle of the day · No. 10 · 22 October
The landlord and the rent
Schulz rents a flat to Lucky and raises the rent by 10% every year. After how many years will it first be more than double the starting rent?
Answer: after 8 years. Interest accrues on interest: 1.1 to the seventh power is 1.95, not yet double; to the eighth it is 2.14. “Ten years” is the mistake of adding percentages, “seven” of trusting the rule of 72, which fibs a little here.
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