Answer:
a) 
b) 6.84 million
c) 2028.52
d) 47 years
Step-by-step explanation:
a)Here we know standard exponential growth rate is

In this put t=0 and a(t)=6.27 million
thus we get a = 6.27 million
now 
this is equal to(in 2012)=
b)Put t = 6
we get

=6.84 million
c) 
take log both sides
we get 16.52 years
d) 
take log both sides we get 47 years to get double the current population.
Ratios don't have to be "per" something, while rates are "per" something.
Ratios compare 2 or more values, as well as rates.
B. Because 5 t 5 t 4 t 4 equals 18
A is your answer tell me if I am wrong.