I think that there is also 4 rows of soprano. This is because is the tenors and sopranos are in seperate rows that means that no tenor or soprano cqn be in that same row. Since each row has 8 people and their is 4 rows of soprno. I multiply 4×8=32. Sice you know that there are 64 singers in total, you subtract 64-32 which will give you 32. Then since there is 8 siners in each row that means 32 divided by8 which is 4.
