Skip to main content
OlympiadHQ

Browse · MATH

Print

jmc

number theory senior

Problem

Remove the integers which are congruent to 3 (mod 7) from the following list of five integers, and sum the integers that remain.
Solution
Recall that if and only if is divisible by 7. Subtracting 3 from every element in the list gives By dividing, we can see that 82 and are not divisible by 7, whereas and are divisible by 7. To see that is not divisible by 7, note that its prime factorization is . So, after striking off the numbers which are congruent to 3 (mod 7), the original list becomes The sum of the remaining integers is .
Final answer
12,\!000,\!085