Explanation
If Rena is the 11th girl from either end of a row of girls, it means there are 10 girls before her from one end and 10 girls after her from the other end. Therefore, the total number of girls in that row is:
10 girls (before Rena) + 1 (Rena) + 10 girls (after Rena) = 21 girls.
So, there are 21 girls in that row.
