Since in 1990 there are 28%, we need to figure out when it gets to 31%. In addition, since it increases by 0.6% every year, we can say that 0.6x+28 (since 28 is the base value) is the percentage of babies born in wedlock every year. Therefore, to get 0.6x+28=31, we subtract 28 from both sides to get 0.6x=3
Dividing both sides by 0.6, we get x=5=the amount of years it takes to get 31% of babies born in wedlock. Since 1990 is the base value (we start from there!), we add 5 to that to get 1990+5=1995 as the yar