The triangular numbers are the numbers 1,3,6,10,15,21,28,36 and so on.
How many of the first 250 triangular numbers are divisible by 5?
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The triangular numbers are the numbers 1,3,6,10,15,21,28,36 and so on.
How many of the first 250 triangular numbers are divisible by 5?
For every 5 triangular numbers, 2 are divisible by 5
So in the first 250 triangular numbers 250/5 * 2 = 100 numbers will be divisible by 5
Yeah. Hari is right.
The statment 'For every 5 triangular numbers, 2 are divisible by 5' make by Hari is the key here. Let me justify that statment
It is obvious that an nth triangular number is the sum of first n natural numbers which is equal to n(n-1)/2). So if a triangular number is divisible by 5 then either n or n-1 is divisible by 5.
There are two such numbers in every 5 triangular numbers which have either n or n-1 divisible by 5.