Answer:
Hamburgers costs $3.50 each.
x- Hamburgers
y- Fries
Shelby: 4x + 2y = $16.50
Samuel: 5x + 3y = $21.25
4x+2y=16.50
-4x -4x
-------------------
2y=-4x+16.50
Divide by 2
y = -2x+8.25
Then use substitution
5x + 3(-2x+8.25) = 21.25
5x -6x +24.75 = 21.25
x=3.50