Complete the following matrix computations.

1. $\left[ \begin{array} { l l } { 1 } & { 2 } \end{array} \right] \left[ \begin{array} { c c } { 2 } & { - 1 } \\ { 4 } & { 0 } \end{array} \right]$ 

   Step (a):

   $\begin{bmatrix}1(2)+2(4)\\1(-1)+2(0)\end{bmatrix}$

2. $\left[ \begin{array} { l l } { 2 } & { 4 } \end{array} \right] \left[ \begin{array} { c c } { 2 } & { - 1 } \\ { 4 } & { 0 } \end{array} \right]$ 

3. Use the results of the pattern of parts (a) and (b) to find the product $\left[ \begin{array} { c c } { 10 } & { 20 } \end{array} \right] \left[ \begin{array} { c c } { 2 } & { - 1 } \\ { 4 } & { 0 } \end{array} \right]$ without computing.

   Hint (c):

   If part (a) is $AB$, then part (b) is $(2A)B$. What is part (c)?

4. Verify your result for part (c) by multiplication.