A company sells trinkets and doodads. It costs the company $30 to make 4 trinkets and 2 doodads and $40 to make 3 trinkets and 4 doodads. How much does it cost to make trinkets and doodads?

tinkets:
[tex]4x = 30 \\ x = 7.5[/tex]
now the next step
[tex]3x + 4y = 40[/tex]
since you know x, you're able to substitute

[tex]3 \times (7.5) + 4y = 40 \\ 22.5 + 4y = 40 \\ 4y = 17.5 \\ y = 4.375[/tex]
so trinket: $7.5
doodad: $4.375