1993 AIME Problem 6

Attempt Problem 6 of the 1993 AIME below, then check your answer against the professionally curated solution from LIVE by Po-Shen Loh. You can also try the full timed exam, view all 1993 AIME solutions, or check the answer key.

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6.

What is the smallest positive integer that can be expressed as the sum of nine consecutive integers, the sum of ten consecutive integers, and the sum of eleven consecutive integers?

Answer: 495
Concepts:arithmetic sequenceleast common multiplemodular arithmetic
Difficulty rating: 1740
Small Hint:

A sum of an odd number of consecutive integers is divisible by the number of terms

Big Hint:

A sum of ten consecutive integers is congruent to 5(mod10)5\pmod {10}

Solution:

The sums of 99 and 1111 consecutive integers are divisible by 99 and 11,11, so the desired number is a multiple of 99.99. A sum of 1010 consecutive integers has the form 10a+45,10a+45, hence is congruent to 5(mod10).5\pmod {10}. The first multiple of 9999 ending in 55 is 599=495,5\cdot99=495, and each of the three required representations then exists.

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