Let 'x' and 'y' be the cost of one rose bush and one bunch of ornamental grass.
Given that Castel paid $69 for 5 rose bushes and 8 bunches of grass,
![5x+8y=69\ldots(1)](https://img.qammunity.org/2023/formulas/mathematics/college/puryebn7l9pk19sylgekgkuwz6qmy93ozd.png)
Also, given that Sumalee paid $42 for 2 rose bushed and 8 bunches of grass,
![2x+8y=42\ldots(2)](https://img.qammunity.org/2023/formulas/mathematics/college/n6xcrclv9wzkf6d1m1eyx3uwtsqbjmzidu.png)
Now that we have two equations and two variables. These can be solved using the Elimination Method.
Subtract equation (2) from (1) as follows,
![\begin{gathered} (5x+8y)-(2x+8y)=69-42 \\ 5x+8y-2x-8y=27 \\ 3x+0=27 \\ x=(27)/(3) \\ x=9 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/s4ji92p413xli2dty9rryw2zei9vn8gdf1.png)
Substitute this value in (1) and obtain the corresponding y-value,
![\begin{gathered} 5(9)+8y=69 \\ 45+8y=69 \\ 8y=69-45 \\ y=(69-45)/(8) \\ y=3 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/vqqb1y1p0byn891tludp1o5o9ygbyaj5z5.png)
So the simultaneous solution is obtained as,
![\begin{gathered} x=9 \\ y=3 \end{gathered}](https://img.qammunity.org/2023/formulas/mathematics/college/hoy2so2u0x6nnwx570b3qr0xgidf966sti.png)
Thus, the cost of one rose bush is $9 and the cost of one bunch of ornamental grass is $3.