2 4 8 16......
the sequence represents geometric progression with first term a = 2 and common ratio r = 2
sum of geometric progression = S = a(r^n-1)/(r-1), where n is number of terms
n =12
S = 2( 2^12 - 1)/ (2-1)
S = 8190
total rabbits by the 12 month = 8190