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---
id: 5900f3b31000cf542c50fec6
challengeType: 5
title: 'Problem 71: Ordered fractions'
---
## Description
< section id = 'description' >
Consider the fraction, n/d, where n and d are positive integers. If n< d and HCF ( n , d ) = 1 , it is called a reduced proper fraction .
If we list the set of reduced proper fractions for d ≤ 8 in ascending order of size, we get:
1/8, 1/7, 1/6, 1/5, 1/4, 2/7, 1/3, 3/8, 2/5, 3/7, 1/2, 4/7, 3/5, 5/8, 2/3, 5/7, 3/4, 4/5, 5/6, 6/7, 7/8
It can be seen that 2/5 is the fraction immediately to the left of 3/7.
By listing the set of reduced proper fractions for d ≤ 1,000,000 in ascending order of size, find the numerator of the fraction immediately to the left of 3/7.
< / section >
## Instructions
< section id = 'instructions' >
< / section >
## Tests
< section id = 'tests' >
```yml
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tests:
- text: < code > euler71()</ code > should return 428570.
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testString: assert.strictEqual(euler71(), 428570, '< code > euler71()< / code > should return 428570.');
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```
< / section >
## Challenge Seed
< section id = 'challengeSeed' >
< div id = 'js-seed' >
```js
function euler71() {
// Good luck!
return true;
}
euler71();
```
< / div >
< / section >
## Solution
< section id = 'solution' >
```js
// solution required
```
< / section >