Answer: 
Step-by-step explanation:
We have the following information:
Fredburguer
, milkshake
and an order of fries
costs
:
(1)
One Fredburguer is equal to a milkshake plus orders of fries:
(2)
Three milshakes is equal to a Fredburguer plus an order of fries:
(3)
And we are asked: If we have a Fredburguer, a milkshake and two orders of fries, how much do we have to pay?
(4)
Well with the first three equations we have a system of equations to solve. Let's begin by isolating
from (3):
(5)
Making (2)=(5):
(6)
Isolating
:
(7)
Substituting (7) in (2):
(8)
Isolating
:
(9)
Substituting (7) and (9) in (1):
(10)
Isolating
:
(11) This is the cost of a Milkshake
Substituting (11) in (7):
(12)
(13) This is the cost of an order of frieds
Substituting (11) in (9):
(14)
(15) This is the cost of a Fredburguer
Substituting (11), (13) and (15) in (4):
(16)
Finally:
This is the cost of a Fredburguer, a milkshake and two orders of fries