Answer:
Step-by-step explanation:
y = yo b^x in year 0 y = yo = 640
640 = 640 b^0 then in year 1 y = 800 and yo = 800
800 = 640 b^1 shows b = 800/640 = 1.25
so your equation becomes
y = 640 (1.25)^x where 'y' is the population in 'x' years
find when population = 1250 :
1250 = 640 (1.25)^x
1250 / 640 = 1.25^x
log (1250/640) = x log 1.25
x = log (1250/640) / log(1.25) = 3 years