Question 1163802
The number of people in each group can be represented by an arithmetic progression, with the first term equal to 1 
and the common difference equal to 2.
The nth term of the progression is: a_n = 1 + 2(n-1)
Thus in the 20th group, there are a_20 = 1 + 2(19) = 39 people
The total number of people is the cumulative sum which is given by:
S_n = (n/2)*(a_1 + a_n) -> S_20 = 10*(1 + 39) = 400
3 hours before, there are a total of 2*9 = 18 groups in line
S_18 = 9*(1+35) = 324