Difference between revisions of "2020 AMC 8 Problems/Problem 7"
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==Video Solution== | ==Video Solution== | ||
+ | https://youtu.be/SPNobOd4t1c (Includes all the problems and has a free class update) | ||
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https://youtu.be/61c1MR9tne8 | https://youtu.be/61c1MR9tne8 | ||
Revision as of 16:47, 14 December 2020
Contents
Problem
How many integers between and
have four distinct digits arranged in increasing order? (For example,
is one integer.)
Solution 1
Firstly, observe that the second digit of such a number cannot be or
, because the digits must be distinct and increasing. The second digit also cannot be
as the number must be less than
, so it must be
. It remains to choose the latter two digits, which must be
distinct digits from
. That can be done in
ways; there is then only
way to order the digits, namely in increasing order. This means the answer is
.
Solution 2 (without using the "choose" function)
As in Solution 1, we find that the first two digits must be , and the third digit must be at least
. If it is
, then there are
choices for the last digit, namely
,
,
,
, or
. Similarly, if the third digit is
, there are
choices for the last digit, namely
,
,
, and
; if
, there are
choices; if
, there are
choices; and if
, there is
choice. It follows that the total number of such integers is
.
Video Solution
https://youtu.be/SPNobOd4t1c (Includes all the problems and has a free class update)
See also
2020 AMC 8 (Problems • Answer Key • Resources) | ||
Preceded by Problem 6 |
Followed by Problem 8 | |
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 AJHSME/AMC 8 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.