The country a consumer price index is approximated by the following formula, where t represents the number of years after 1990.
a(t)=100e^0.023t
for instance, since a(16) is about 144, the amount of goods that could be purchased for $100 in 1990 cost about $144 in 2006. use the function to determine the year during which costs will be 90% higher than in 1990.