Answer:
Small pizza is for 1 person, Large pizza is for 5 people
Skills needed: System of Equations
Explanation:
1) First, let's make two variables:
--> = how many people a large pizza can feed
--> = how many people a small pizza can feed
2) We need to use a system of equations to solve this --> A system is 2 or more equations with specific solutions for the variables involved.
Example:
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We can solve with elimination:
(Rewriting the first equation to look like the second one)
---> Then we add both:
When we add both equations, we eliminate the variable , solving for easily. After this, we can solve for
---> 
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This is the elimination method of a system (Note: we can also subtract equations if needed)
3) Let's make a system for this problem:
 ---> 3 small pizzas (multiplied by the number of people it can feed) + 2 large pizzas (multiplied by the number of people it can feed) is for 13 people.
 ---> 3 small pizzas (multiplied by the number of people it can feed) + 4 large pizzas (multiplied by the number of people it can feed) is for 23 people.
4) We have two equations:
We can eliminate by subtracting both equations.
---> 
We subtracted from which is just 0 (so it does not have to be shown)
We subtracted from which is just 
On the right side: subtracted from which is just 
----> , divide by -2, 
, which means a large pizza is for 5 people.
Substituting into the first equation (it does not matter which one you substitute into) will get 
Small pizza is for 1 person.