Arithmetic progression's summation formula is s= (the first term+the last term) * the number of terms and then divided by 2 (the proof is abbreviated).
Suppose the number of people in the first row is a 1, then the last item is at least a 16.
Because a 16=a 1+ 15, there is (a1+a1+15) *16/2 =/kloc-.
The solution is a 1=55,
So there are 55 people in the first row.
I hope I have the right question, or that's it.