Assume that the cost of the milk is x.
We are given that a sandwich costs 0.75$ more than the cost of milk. This means that:
cost of sandwich = cost of milk + 0.75
cost of sandwich = x + 0.75
Now, the customer bought 4 sandwiches and 4 containers of milk at the cost of 7$.
We will transform this into an equation as follows:
total cost = cost of 4 sandwiches + cost of 4 containers of milk
7 = 4(x+0.75) + 4(x)
Solve the above equation to get x as follows:
7 = 4x + 3 + 4x
7-3 = 8x
8x = 4
x = 0.5
Therefore:
cost of milk = x = $0.5
cost of sandwich = x+0.75 = 0.5+0.75 = $1.25