Let the cost of soda be s.
Let the cost of hamburgers = h
[tex]\begin{cases} &2s + 4h = 12 ----- (1) \\&4s + 2h = 9 ----- (2)\end{cases}[/tex]
Multiply Equation (1) by 2 :
[tex]\begin{cases} &4s + 8h = 24 ----- (1)\\&4s + 2h = 9 ----- (2)\end{cases}[/tex]
Equation (1) - (2):
[tex]\begin{aligned} &6h = 15 \\&h = 2.5 \end{aligned} [/tex]
Answer: 1 hamburger costs $2.50