freeCodeCamp/curriculum/challenges/english/10-coding-interview-prep/project-euler/problem-305-reflexive-position.english.md

---
id: 5900f49d1000cf542c50ffb0
challengeType: 5
isHidden: false
title: 'Problem 305: Reflexive Position'
forumTopicId: 301959
---

## Description
<section id='description'>
Let's call S the (infinite) string that is made by concatenating the consecutive positive integers (starting from 1)  written down in base 10.
Thus, S = 1234567891011121314151617181920212223242...


It's easy to see that any number will show up an infinite number of times in S.


Let's call f(n) the starting position of the nth occurrence of n in S.
For example, f(1)=1, f(5)=81, f(12)=271 and f(7780)=111111365.


Find ∑f(3k) for 1≤k≤13.
</section>

## Instructions
<section id='instructions'>

</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: <code>euler305()</code> should return 18174995535140.
    testString: assert.strictEqual(euler305(), 18174995535140);

```

</section>

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

<div id='js-seed'>

```js
function euler305() {
  // Good luck!
  return true;
}

euler305();
```

</div>



</section>

## Solution
<section id='solution'>

```js
// solution required
```

</section>