---
id: 5900f45f1000cf542c50ff71
challengeType: 5
title: 'Problem 242: Odd Triplets'
forumTopicId: 301889
---

## Description
<section id='description'>
Given the set {1,2,...,n}, we define f(n,k) as the number of its k-element subsets with an odd sum of elements. For example, f(5,3) = 4, since the set {1,2,3,4,5} has four 3-element subsets having an odd sum of elements, i.e.: {1,2,4}, {1,3,5}, {2,3,4} and {2,4,5}.

When all three values n, k and f(n,k) are odd, we say that they make
an odd-triplet [n,k,f(n,k)].

There are exactly five odd-triplets with n ≤ 10, namely:
[1,1,f(1,1) = 1], [5,1,f(5,1) = 3], [5,5,f(5,5) = 1], [9,1,f(9,1) = 5] and [9,9,f(9,9) = 1].

How many odd-triplets are there with n ≤ 1012 ?
</section>

## Instructions
<section id='instructions'>

</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: <code>euler242()</code> should return 997104142249036700.
    testString: assert.strictEqual(euler242(), 997104142249036700);

```

</section>

## Challenge Seed
<section id='challengeSeed'>

<div id='js-seed'>

```js
function euler242() {
  // Good luck!
  return true;
}

euler242();
```

</div>



</section>

## Solution
<section id='solution'>

```js
// solution required
```

</section>