15.rb

move_array = []
20.times { move_array << 'r' }
20.times { move_array << 'd' }

puts move_array.combination(20).to_a
class Array
  def solutions
    x=0
    y=0
    temp=self
    until temp.empty?
      current=temp.pop
      if current == r
        x+=1
      elsif current == d
        y-+1
      end  
    end
    if x == 20 && y == -20
      true
    else
      false
    end
  end
end