ACT Practice Question: Linear System of Equations

 
Here is an ACT math practice question on solutions of linear system of equations.

$$ x – 2y = 4 $$
$$ 2x + my = 8 $$
For what value of $m$ will the system of equations above have
an infinite number of solutions?

1. $\quad -4$
2. $\quad -2$
3. $\quad 2$
4. $\quad 4$
5. $\quad 8$