Percent Error - Definition & Examples

In $240$ B.C., over $2250$ years ago, Eratosthenes was the first person to calculate the circumference of the Earth. Due to not knowing exactly what units he used, Eratosthenes could have been anywhere from $0.5\%$ to $17\%$ off. The circumference of the Earth is $24,901$ miles. If Eratosthenes undershot by $17\%$, what would his measurement have been in miles?