# Difference between revisions of "2005 AMC 10A Problems/Problem 21"

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==Problem== | ==Problem== | ||

− | For how many positive integers <math>n</math> does <math> 1+2+...+n </math> evenly divide <math>6n</math>? | + | For how many positive integers <math>n</math> does <math> 1+2+...+n </math> evenly divide from <math>6n</math>? |

<math> \mathrm{(A) \ } 3\qquad \mathrm{(B) \ } 5\qquad \mathrm{(C) \ } 7\qquad \mathrm{(D) \ } 9\qquad \mathrm{(E) \ } 11 </math> | <math> \mathrm{(A) \ } 3\qquad \mathrm{(B) \ } 5\qquad \mathrm{(C) \ } 7\qquad \mathrm{(D) \ } 9\qquad \mathrm{(E) \ } 11 </math> |

## Revision as of 23:38, 24 January 2008

## Problem

For how many positive integers does evenly divide from ?

## Solution

If evenly divides , then is an integer.

Since we may substitute the RHS in the above fraction.

So the problem asks us for how many positive integers is an integer.

is an integer when is a factor of .

The factors of are , , , , , and .

So the possible values of are , , , , , and .

But isn't a positive integer, so only , , , , and are possible values of .

Therefore the number of possible values of is