How do you write and solve a system of equations from a word problem?

See this question for an example.
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Step 1. Read and understand the description of the problem.
Step 2. Identify what the problem is asking you do do.
Step 3. Identify the given information and the given relationships.
Step 4. Use your knowledge of the relationships associated with the parts of the problem, along with the result of Step 3, to write equations relating the given information and the information you are asked to find.
Step 5. Solve those equations using the rules of algebra.
Step 6. Check to make sure your solution satisfies all of the problem conditions and requests.


With respect to the example problem above, you are expected to know how the total bill is calculated when you make a purchase. A lot of people act like they don't understand how this works, even though they have been shopping many times. This is what is meant by Step 4--use your knowledge.