A statistical study showed that since 1990, the number of households with a computer in a city is given by the following formula : f (t ) = ...., where  t  is the number of years elapsed since 1990.
We suppose that f (t) is a good approximation, at least until 2010, of the number of households equipped with a computer in this city.
Find an approximate value of the average percentage of households equipped with a computer since the beginning of 1997 until the beginning of 1999.