Linear AlgebraEasyRow Reduction & Rank

Row Reduction & Rank · Easy problem

A matrix in echelon form is

\begin{pmatrix} 1 & 4 & 0 & 2 & 1 \\ 0 & 0 & 3 & 1 & 0 \\ 0 & 0 & 0 & 0 & 7 \end{pmatrix}.

The columns are numbered $1$ to $5$ . Find the sum of the numbers of the columns that do NOT contain a pivot (the free columns).