2017 AMC 10B Problems/Problem 13
Contents
Problem
There are 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
students taking yoga,
taking bridge, and
taking painting. There are
students taking at least two classes. How many students are taking all three classes?
Solution 1
By PIE (Property of Inclusion/Exclusion), we have
Number of people in at least two sets is
So,
which gives
Solution 2 (Subtraction)
The total number of classes taken among the 20 students is . Each student is taking at least one class so let's subtract the
classes (
per each of the
students) from
classes to get
.
classes is the total number of extra classes taken by the students who take
or
classes. Since we know that there are
students taking at least
classes, there must be
students that are taking all
classes.
Solution 3 (Algebra)
Total class count is 32. Assume there are students taking one class,
students taking two classes, ad
students taking three classes. Because there are
students total,
. Because each student taking two classes is counted twice, and each student taking three classes is counted thrice in the total class count,
. There are
students taking two or three classes, so
. Solving this system of equations gives us
.
Solution 4 (Venn Diagrams and Algebraic Substitution)
Let us assign the following variables and put them in our Venn Diagram:
which designates the number of people taking exactly Bridge and Yoga.
which designates the number of people taking exactly Bridge and Painting.
which designates the number of people that took all
classes or what we want to find.
which designates the number of people taking exactly Yoga and Painting.
Note: The Venn Diagram is linked here: https://artofproblemsolving.com/wiki/index.php/File:IMG_20220704_192122676_2.jpg#filelinks
Let's now recall what information we have given: There are exactly people that are taking at least
classes meaning in other words,
people total are taking strictly
classes or strictly all the available classes meaning that
.
Let's now start filling out the Venn Diagram:
Strictly taking Bridge, no other classes: We know in total, the number is , 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,
. Giving our answer as
.
However, this equation seems complicated as it has different variables, so to make this look a lot less complicated we can use our earlier equation:
to see that
. This means that this can also be written as
.
Strictly taking Yoga Only: The total number of people is , 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:
.
Again, we can use substitution to see that . This simplifies our equation to
.
Strictly taking Painting Only: We know again, in total this number is , which also accounts for the people taking other classes too. From our Venn Diagram it is visible that we need to subtract:
giving
.
Again, through substitution of our first equation we see that meaning we can simplify this equation to
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 students in total, and
students are taking exactly
or
classes, meaning that if we do
we get our answer for the number of students taking exactly
class because those taking exactly one class have no overlap with those taking exactly
or exactly
classes as shown in our Venn Diagram and because
,
, and
classes are subcases for finding the total number of students as we know that each student is in exactly
, or
classes. This means that exactly
students took strictly
class.
We can add up our equations we found to equal because those equations were for subcases of having exactly
class giving:
.
.
From our equation , we can substitute
for
giving us:
.
This gives . We assigned
for the number of people taking exactly
classes meaning that when we find
, we find the answer. This means our answer is
.
~Batmanstark
See Also
2017 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 12 |
Followed by Problem 14 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
All AMC 10 Problems and Solutions |
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