Explanation:

Successive Percentage Change for 2 years is given by:-   (A+B+AB/100)%

Where  'A' is the first change and 'B' is the second. Let's put these values in the above formula.

[-12 + 15 + (-12)(15)/100] %             (Here A = -12 and B = +15)

[3 - 1.8] % = 1.2%

Since it is a positive change,Hence the population will be increased by 1.2% in the starting of 1990 (Or in the end of 1989).