Difference between revisions of "2020 AMC 10A Problems/Problem 18"
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| − | In order for <math>a\cdot d-b\cdot c</math> to be odd, consider parity. We must have even | + | In order for <math>a\cdot d-b\cdot c</math> to be odd, consider parity. We must have an even number minus an odd number or an odd number minus an even number. |
==See Also== | ==See Also== | ||
Revision as of 21:55, 31 January 2020
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
In order for to be odd, consider parity. We must have an even number minus an odd number or an odd number minus an even number.
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 | ||
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