If the interest rate is 5% per annum, how long will it take to double your money?
How long will it take to triple it? If the interest rate is 8% per annum, how long will it take to double your money?
From Fibonacci's Liber Abaci, written in the year 1202: "A certain man gave 1 denaro at interest so that in 5 years he must receive double the denari, and in another 5, he must have double 2 of the denari and thus forever. How many denari from this 1 denaro must he have in 100 years?"