a. 4(1.09)^n price of hamburger at n year.
b. cost of hamburger after 2 years = 4(1.09)^2 = $4.75 approx
c. since increasement takes place every year (at the begining of the year )and lasts for the whole year
if we calculate the price 4 years from now, the price will be the same for the next 6 months.
so price for 4 years and 6 months from now; 4(1.09)^4 = $5.65 approx