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Junior Mathematical Olympiad

Netherlands number theory

Problem

A triangular number is the number of circles you can place in an equilateral triangle of a certain height. For example, in the figure on the right, you can see that the fourth triangular number is . Jasmine writes down the first triangular numbers: , , , , , . How many of these numbers end in a ?
problem
Solution
6.
Final answer
200

Techniques

Modular ArithmeticDivisibility / Factorization