Another function often used with data collections is one which groups the elements according to some evaluation result.

Consider the following example. Let's say you have a list of 100 integers and you want to group them according to their even and odd value.

In Ruby, you can easily do this by using group_by method provide by Enumerable module.

> (1..5).group_by {|x| x%2}
{1=>[1,3,5], 0=>[2, 4]}

Of course this is a very simple example and its use can be very complicated in nature.

In this challenge, your task is to group the students into two categories corresponding to their marks obtained in a test. The list of students will be provided in a marks hash with student name as key and marks obtained (out of 100) as value pair, along with the pass_marks as argument.

The method group_by_marks must return a Hash containing an array of students who passed as value to Passed key, and those who failed as value to Failed key. If a particular key is empty, don't return that key.

For example,

> marks = {"Ramesh":23, "Vivek":40, "Harsh":88, "Mohammad":60}
> group_by_marks(marks, 30)
=> {"Failed"=>[["Ramesh", 23]], "Passed"=>[["Vivek", 40], ["Harsh", 88], ["Mohammad", 60]]}