Difference between revisions of "2019 AMC 10A Problems/Problem 20"
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==Solution== | ==Solution== | ||
Note that odd sums can only be formed by <math>(e,e,o)</math> or <math>(o,o,o),</math> so we focus on placing the evens to get <math>\frac{5! \cdot 4! \cdot 9}{9!}=\boxed{B) \frac{1}{14}}</math> (we need to have each even be with another even in each row/column) | Note that odd sums can only be formed by <math>(e,e,o)</math> or <math>(o,o,o),</math> so we focus on placing the evens to get <math>\frac{5! \cdot 4! \cdot 9}{9!}=\boxed{B) \frac{1}{14}}</math> (we need to have each even be with another even in each row/column) | ||
| + | |||
| + | ==Solution 2== | ||
| + | By the Pigeonhole Principle, there must be at least one row with 2 or more odd numbers in it. Therefore, that row must contain three odd numbers in order to sum to odd. The same thing can be done with the columns. Then we simply have to choose one row and one column to be filled with odd numbers - which can happen <math>3\cdot 3 = 9</math> times. The answer is then | ||
| + | <cmath>\frac{9}{\binom{9}{4}}=\boxed{\text{(B) }\tfrac{1}{14}}</cmath> | ||
| + | |||
| + | -programjames1 | ||
==See Also== | ==See Also== | ||
Revision as of 18:28, 9 February 2019
- The following problem is from both the 2019 AMC 10A #20 and 2019 AMC 12A #16, so both problems redirect to this page.
Contents
Problem
The numbers 
are randomly placed into the 
squares of a 
grid. Each square gets one number, and each of the numbers is used once. What is the probability that the sum of the numbers in each row and each column is odd?
Solution
Note that odd sums can only be formed by 
or 
so we focus on placing the evens to get 
(we need to have each even be with another even in each row/column)
Solution 2
By the Pigeonhole Principle, there must be at least one row with 2 or more odd numbers in it. Therefore, that row must contain three odd numbers in order to sum to odd. The same thing can be done with the columns. Then we simply have to choose one row and one column to be filled with odd numbers - which can happen 
times. The answer is then
![\[\frac{9}{\binom{9}{4}}=\boxed{\text{(B) }\tfrac{1}{14}}\]](http://latex.artofproblemsolving.com/1/d/5/1d573310eb5f1d65389ba79939e968a331c4c35a.png)
-programjames1
See Also
| 2019 AMC 10A (Problems • Answer Key • Resources) | ||
| Preceded by Problem 19 |
Followed by Problem 21 | |
| 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 | ||
| 2019 AMC 12A (Problems • Answer Key • Resources) | |
| Preceded by Problem 15 |
Followed by Problem 17 |
| 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 12 Problems and Solutions | |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions. 
