Question 65739
Parents=2
Grand parents=4=2^2
"Great" starts with  2^3  people
Each generation before that multiplies by 2
The last generation you want has 2^3*2^39=2^42
which is a very large number.
Comment: I can't tell if you are starting the count with yourself:
if you are the total number is:
1+2+2^2+2^3+...+2^42
Using the sum formula with r=2 and n=41 the sum is:
S(41)=1[2^(42)-1]/[2-1]
=2^42-1
=4.398046511x10^12
Cheers,
stan H.