The current notation of indices is confusing. During our studying matrices, we are used to describing Aij element as i-th row and j-th column.
But in this example we see that, for example, c2 element is the sum of n21 + n22 + n23. Using matrices indices, we'll write something like that:
Column element c2 is equal to the sum of all elements of a matrix in the second column, i.e. c2 = n12 + n22 + n32
And row element r3 is equal to the sum of all elements of a matrix in the third row, i.e. r3 = n31 + n32 + n33 + n34 + n35
The current notation of indices is confusing. During our studying matrices, we are used to describing Aij element as i-th row and j-th column. But in this example we see that, for example, c2 element is the sum of n21 + n22 + n23. Using matrices indices, we'll write something like that:
Column element c2 is equal to the sum of all elements of a matrix in the second column, i.e. c2 = n12 + n22 + n32 And row element r3 is equal to the sum of all elements of a matrix in the third row, i.e. r3 = n31 + n32 + n33 + n34 + n35
Graphical example: https://imgur.com/a/LCC0nVU