number in the 1-st generation n_1=n_0*7=7
number in the 2-nd generation n_2=n_1*7=49
and so on.
So we may evaluate this chain multiplying by 7 for each generation, or we
can observe that this gives us 7^r in the r-th generation.
So the number who have seen it by the r-th generation is the sum of the
number in each of the generations up to and including the r-th:
N_r=n_0 + n_1 + .. + n_r = 1+ 7 + 7^2 + ... + 7^r.
This is a finite geometric series (or progression), who's sum is:
N_r= 1+ 7 + 7^2 + ... + 7^r = (1-7^9)/(1-7) = 6725601.
(you are expected to know that: