if 7 sandwiches, 10 cups of coffee and 1 piece of pie costs $4.20
and 5 sandwiches, 7 cups of coffee and 1 piece of pie costs $3.40.
How much is 1 sandwich, 1 cup of coffee and 1 piece of pie?
Have you tried to form some equations with this information?
Lets called the price of a sandwich = S, the price of a coffee = C and the price of pie = P
Here's a start, you have: