Tickets for a movie cost $6 for children and $12 for adults. If 119 seats were filled, and the box office revenues were $900. How many children's and now many adult tickets were sold?
Let 
be the number of children's tickets and 
be the number of adult tickets. This gives the system of equations
. Writing the system as a matrix gives ![LaTeX: \displaystyle \left[\begin{matrix}1 & 1 & 119\\6 & 12 & 900\end{matrix}\right]](/equation_images/%20%5Cdisplaystyle%20%5Cleft%5B%5Cbegin%7Bmatrix%7D1%20%26%201%20%26%20119%5C%5C6%20%26%2012%20%26%20900%5Cend%7Bmatrix%7D%5Cright%5D%20 "\displaystyle \left[\begin{matrix}1 & 1 & 119\\6 & 12 & 900\end{matrix}\right]")
. Using RREF gives ![LaTeX: \displaystyle \left[\begin{matrix}1 & 0 & 88\\0 & 1 & 31\end{matrix}\right]](/equation_images/%20%5Cdisplaystyle%20%5Cleft%5B%5Cbegin%7Bmatrix%7D1%20%26%200%20%26%2088%5C%5C0%20%26%201%20%26%2031%5Cend%7Bmatrix%7D%5Cright%5D%20 "\displaystyle \left[\begin{matrix}1 & 0 & 88\\0 & 1 & 31\end{matrix}\right]")
. The number of children's tickets is 88 and the number of adult tickets is 31.