Problem: There are $20$ students participating in an after-school program offering classes in yoga, bridge, and painting. Each student must take at least one of these three classes, but may take two or all three. There are $10$ students taking yoga, $13$ taking bridge, and $9$ taking painting. There are $9$ students taking at least two classes. How many students are taking all three classes?
$\textbf{(A)}\ 1\qquad\textbf{(B)}\ 2\qquad\textbf{(C)}\ 3\qquad\textbf{(D)}\ 4\qquad\textbf{(E)}\ 5$

Solution: Let us assign the following variables and put them in our Venn Diagram [1] $a$ which designates the number of people taking exactly Bridge and Yoga. $b$ which designates the number of people taking exactly Bridge and Painting. $c$ which designates the number of people that took all $3$ classes or what we want to find. $d$ which designates the number of people taking exactly Yoga and Painting.
Let's now recall what information we have given: There are exactly $9$ people that are taking at least $2$ classes meaning in other words, $9$ people total are taking strictly $2$ classes or strictly all the available classes meaning that $a+b+c+d=9$
Let's now start filling out the Venn Diagram:
Strictly taking Bridge, no other classes: We know in total, the number is $13$ , however this includes the people taking other classes too meaning we'd need to do some subtraction. From our Venn Diagram we see that we'd need to subtract the following variables to get our wanted outcome here, $a, b, c$ . Giving our answer as $13-(a+b+c)$
However, this equation seems complicated as it has $3$ different variables, so to make this look a lot less complicated we can use our earlier equation: $a+b+c+d=9$ to see that $a+b+c=9-d$ . This means that this can also be written as $13-(9-d)=4+d$
Strictly taking Yoga Only: The total number of people is $10$ , but this would also count people taking other classes too along with it, so we need to subtract this overcount which is visible in the Venn Diagram giving us: $10-(a+c+d)$
Again, we can use substitution to see that $a+c+d=9-b$ . This simplifies our equation to $10-(9-b)=1+b$
Strictly taking Painting Only: We know again, in total this number is $9$ , which also accounts for the people taking other classes too. From our Venn Diagram it is visible that we need to subtract: $b, c, d$ giving $9-(b+c+d)$
Again, through substitution of our first equation we see that $b+c+d=9-a$ meaning we can simplify this equation to $9-(9-a)=a$
If we add these newly made equations of strictly taking one class, we get the total number of people taking exactly one class as these equations each were a subcase for it. We can also find the exact number for this because we are given that there are exactly $20$ students in total, and $9$ students are taking exactly $2$ or $3$ classes, meaning that if we do $20-9$ we get our answer for the number of students taking exactly $1$ class because those taking exactly one class have no overlap with those taking exactly $2$ or exactly $3$ classes as shown in our Venn Diagram and because $1$ $2$ , and $3$ classes are subcases for finding the total number of students as we know that each student is in exactly $1, 2$ , or $3$ classes. This means that exactly $11$ students took strictly $1$ class.
We can add up our equations we found to equal $11$ because those equations were for subcases of having exactly $1$ class giving: $(4+d)+(1+b)+a=11$ $=5+a+b+d=11$
From our equation $a+b+c+d=9$ , we can substitute $9-c$ for $a+b+d$ giving us: $5+9-c=11$
This gives $c=3$ . We assigned $c$ for the number of people taking exactly $3$ classes meaning that when we find $c$ , we find the answer. This means our answer is $\boxed{3}$

Answer: C 3