This cache is the physical bonus for the River Quaggy Parks Adventure Lab series. Wheelchair users may find some elements of the Adventure Lab caches difficult to access, but this physical cache is a little inaccessible.
To find the Bonus you'll need to complete the Adventure Lab series collecting the bonus info from each stage and using those numbers to calculate the formula.
Cache can be found at N51 27.ABC W000 00.DEF
The Difficulty rating reflects that you have to find the five Lab caches first. The difficulty of the cache is much lower.