### Percentile

Use `Array.reduce()` to calculate how many numbers are below the value and how many are the same value and
apply the percentile formula.

```js
const percentile = (arr, val) => 
  100 * arr.reduce((acc,v) => acc + (v < val ? 1 : 0) + (v === val ? 0.5 : 0), 0) / arr.length;
// percentile([1,2,3,4,5,6,7,8,9,10], 6) -> 55
 ```