freeCodeCamp/curriculum/challenges/english/08-coding-interview-prep/project-euler/problem-145-how-many-reversible-numbers-are-there-below-one-billion.english.md

---
id: 5900f3fd1000cf542c50ff10
challengeType: 5
title: 'Problem 145: How many reversible numbers are there below one-billion?'
---

## Description
<section id='description'>
Some positive integers n have the property that the sum [ n + reverse(n) ] consists entirely of odd (decimal) digits. For instance, 36 + 63 = 99 and 409 + 904 = 1313. We will call such numbers reversible; so 36, 63, 409, and 904 are reversible. Leading zeroes are not allowed in either n or reverse(n).

There are 120 reversible numbers below one-thousand.

How many reversible numbers are there below one-billion (109)?
</section>

## Instructions
<section id='instructions'>

</section>

## Tests
<section id='tests'>

```yml
tests:
  - text: <code>euler145()</code> should return 608720.
    testString: assert.strictEqual(euler145(), 608720, '<code>euler145()</code> should return 608720.');

```

</section>

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

<div id='js-seed'>

```js
function euler145() {
  // Good luck!
  return true;
}

euler145();
```

</div>



</section>

## Solution
<section id='solution'>

```js
// solution required
```
</section>