completed array_problems/arrays.rb

I also added a method to eliminate non-number elements of array and called that at the beginning of each of the three methods.
I would start with a array[0] here as your max number. You could potentially have an array that looks like this:
[-10000, -9999, -5000, -1000000]
and the method would currently return -1000 instead of the correct value of -5000.