Difference between revisions of "2022 AIME II Problems/Problem 14"
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For positive integers <math>a</math>, <math>b</math>, and <math>c</math> with <math>a < b < c</math>, consider collections of postage stamps in denominations <math>a</math>, <math>b</math>, and <math>c</math> cents that contain at least one stamp of each denomination. If there exists such a collection that contains sub-collections worth every whole number of cents up to <math>1000</math> cents, let <math>f(a, b, c)</math> be the minimum number of stamps in such a collection. Find the sum of the three least values of <math>c</math> such that <math>f(a, b, c) = 97</math> for some choice of <math>a</math> and <math>b</math>. | For positive integers <math>a</math>, <math>b</math>, and <math>c</math> with <math>a < b < c</math>, consider collections of postage stamps in denominations <math>a</math>, <math>b</math>, and <math>c</math> cents that contain at least one stamp of each denomination. If there exists such a collection that contains sub-collections worth every whole number of cents up to <math>1000</math> cents, let <math>f(a, b, c)</math> be the minimum number of stamps in such a collection. Find the sum of the three least values of <math>c</math> such that <math>f(a, b, c) = 97</math> for some choice of <math>a</math> and <math>b</math>. | ||
− | ==Solution | + | ==Solution== |
Notice that we must have <math>a = 1</math>, otherwise <math>1</math> cent stamp cannot be represented. At least <math>b-1</math> numbers of <math>1</math> cent stamps are needed to represent the values less than <math>b</math>. Using at most <math>c-1</math> stamps of value <math>1</math> and <math>b</math>, it can have all the values from <math>1</math> to <math>c-1</math> cents. Plus <math>\lfloor \frac{999}{c} \rfloor</math> stamps of value <math>c</math>, every value up to <math>1000</math> can be represented. Therefore using <math>\lfloor \frac{999}{c} \rfloor</math> stamps of value <math>c</math>, <math>\lfloor \frac{c-1}{b} \rfloor</math> stamps of value <math>b</math>, and <math>b-1</math> stamps of value <math>1</math>, all values up to <math>1000</math> can be represented in sub-collections, while minimizing the number of stamps. | Notice that we must have <math>a = 1</math>, otherwise <math>1</math> cent stamp cannot be represented. At least <math>b-1</math> numbers of <math>1</math> cent stamps are needed to represent the values less than <math>b</math>. Using at most <math>c-1</math> stamps of value <math>1</math> and <math>b</math>, it can have all the values from <math>1</math> to <math>c-1</math> cents. Plus <math>\lfloor \frac{999}{c} \rfloor</math> stamps of value <math>c</math>, every value up to <math>1000</math> can be represented. Therefore using <math>\lfloor \frac{999}{c} \rfloor</math> stamps of value <math>c</math>, <math>\lfloor \frac{c-1}{b} \rfloor</math> stamps of value <math>b</math>, and <math>b-1</math> stamps of value <math>1</math>, all values up to <math>1000</math> can be represented in sub-collections, while minimizing the number of stamps. |
Revision as of 02:17, 13 June 2022
Problem
For positive integers ,
, and
with
, consider collections of postage stamps in denominations
,
, and
cents that contain at least one stamp of each denomination. If there exists such a collection that contains sub-collections worth every whole number of cents up to
cents, let
be the minimum number of stamps in such a collection. Find the sum of the three least values of
such that
for some choice of
and
.
Solution
Notice that we must have , otherwise
cent stamp cannot be represented. At least
numbers of
cent stamps are needed to represent the values less than
. Using at most
stamps of value
and
, it can have all the values from
to
cents. Plus
stamps of value
, every value up to
can be represented. Therefore using
stamps of value
,
stamps of value
, and
stamps of value
, all values up to
can be represented in sub-collections, while minimizing the number of stamps.
So, ,
. We can get the answer by solving this equation.
,
or
,
For
,
,
![]()
,
![]()
For
,
![]()
,
![]()
,
, no solution
![]()
,
![]()
,
or
, neither values satisfy
, no solution
![]()
,
![]()
,
![]()
![]()
,
![]()
,
![]()
The least values of
is
,
,
.
See Also
2022 AIME II (Problems • Answer Key • Resources) | ||
Preceded by Problem 13 |
Followed by Problem 15 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 | ||
All AIME Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.