Difference between revisions of "2020 AMC 10A Problems/Problem 18"
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To get an even products, we count: <math>\text{P(any number)} \cdot \text{P(any number)}-\text{P(odd)}\cdot\text{P(odd)}</math>, which is <math>4 \cdot 4 - 2 \cdot 2=12</math>. | To get an even products, we count: <math>\text{P(any number)} \cdot \text{P(any number)}-\text{P(odd)}\cdot\text{P(odd)}</math>, which is <math>4 \cdot 4 - 2 \cdot 2=12</math>. | ||
The number of ways to get an odd product can be counted like so: <math>\text{P(odd)}\cdot\text{P(odd)}</math>, which is <math>2 \cdot 2</math>, or <math>4</math>. | The number of ways to get an odd product can be counted like so: <math>\text{P(odd)}\cdot\text{P(odd)}</math>, which is <math>2 \cdot 2</math>, or <math>4</math>. | ||
− | So, for one product to be odd the other to be even: <math>2 \cdot 4 \cdot 12=\boxed{(C)96}</math>(order matters). ~ Anonymous and Arctic_Bunny | + | So, for one product to be odd the other to be even: <math>2 \cdot 4 \cdot 12=\boxed{(C)96}</math>(order matters). |
+ | ~ Anonymous and Arctic_Bunny | ||
===Solution 4=== | ===Solution 4=== |
Revision as of 21:21, 2 February 2020
Contents
Problem
Let be an ordered quadruple of not necessarily distinct integers, each one of them in the set
For how many such quadruples is it true that
is odd? (For example,
is one such quadruple, because
is odd.)
Solution
Solution 1
In order for to be odd, consider parity. We must have (even)-(odd) or (odd)-(even). There are
ways to pick numbers to obtain an even product. There are
ways to obtain an odd product. Therefore, the total amount of ways to make
odd is
.
-Midnight
Solution 2
Consider parity. We need exactly one term to be odd, one term to be even. Because of symmetry, we can set to be odd and
to be even, then multiply by
If
is odd, both
and
must be odd, therefore there are
possibilities for
Consider
Let us say that
is even. Then there are
possibilities for
However,
can be odd, in which case we have
more possibilities for
Thus there are
ways for us to choose
and
ways for us to choose
Therefore, also considering symmetry, we have
total values of
Solution 3
There are 4 ways to choose any number independently and 2 ways to choose any odd number independently.
To get an even products, we count: , which is
.
The number of ways to get an odd product can be counted like so:
, which is
, or
.
So, for one product to be odd the other to be even:
(order matters).
~ Anonymous and Arctic_Bunny
Solution 4
We use complementary counting: If the difference is even, then we can subtract those cases. There are a total of cases.
For an even difference, we have (even)-(even) or (odd-odd).
From Solution 3:
"There are 4 ways to choose any number independently and 2 ways to choose any odd number independently.
even products:(number)*(number)-(odd)*(odd): .
odd products: (odd)*(odd):
."
With this, we easily calculate .
~kevinmathz
Video Solution
~IceMatrix
Additional Note
When calculating the number of even products and odd products, since the only way to get an odd product is to multiply two odd integers together, and there are odd integers, it can quickly be deduced that there are
possibilities for an odd product. Since the product must be either odd or even, and there are
ways to choose factors for the product, there are
possibilities for an even product. ~emerald_block
See Also
2020 AMC 10A (Problems • Answer Key • Resources) | ||
Preceded by Problem 17 |
Followed by Problem 19 | |
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 |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.