For this case we propose a system of equations:
x: Let the variable representing the cost of a milkshake
y: Let the variable representing the cost of a burger
According to the statement we have:
![2x + 3y = 13\\5x + 7y = 31](https://img.qammunity.org/2020/formulas/mathematics/middle-school/paygo0evenx8dg0ka8abhchfu5qxgaeywi.png)
We multiply the first equation by -5:
![-10x-15y = -65](https://img.qammunity.org/2020/formulas/mathematics/middle-school/prlc7wi3czgy3tbgc5hj0y1bh44bmydl38.png)
We multiply the second equation by 2:
![10x + 14y = 62](https://img.qammunity.org/2020/formulas/mathematics/middle-school/tucmcq6c53a1n8k53366kot5t5ha6unb4j.png)
We have the following equivalent system:
![-10x-15y = -65\\10x + 14y = 62](https://img.qammunity.org/2020/formulas/mathematics/middle-school/jvlim7x1xp3obg1gx0ruid5gwtb12euc8u.png)
We add the equations:
![-10x + 10x-15y + 14y = -65 + 62\\-y = -3\\y = 3](https://img.qammunity.org/2020/formulas/mathematics/middle-school/j8jrsoajfmx52h55daufsuxd10tlfbef3h.png)
Thus, the cost of a burger is $3.
![2x + 3 (3) = 13\\2x + 9 = 13\\2x = 13-9\\2x = 4\\x = \frac {4} {2}\\x = 2](https://img.qammunity.org/2020/formulas/mathematics/middle-school/3iykr5n3munywztbh4zbxvg4fq1lvulw17.png)
So, the cost of a milkshake is $2
Answer:
Burger: $3
Milkshake: $2