list_statistics.rb

list = [75, 100, 85, 65, 84, 87, 95]

def highest(list_given)
  maximum = 0
  list_given.each do |item|
    if item > maximum
      maximum = item
    end
  end
  return maximum
end

def lowest(list_given)
  minimum = list_given[0]
  list_given.each do |item|
    if item < minimum
      minimum = item
    end
  end
  return minimum
end

def add_up_list(list_given)
  total = 0
  list_given.each do |item|
    total += item
  end
  total
end

def list_size(list_given)
  size = 0
  list_given.each do |item|
    size += 1
  end
  size
end

def average(sum, list_given_size)
  sum / list_given_size
end

puts "Highest: #{highest(list)}"
puts "Lowest: #{lowest(list)}"
puts "Average: #{average(add_up_list(list), list_size(list))}"

Be careful about leaving trailing whitespace in your files. You've got a blank space on the lines that are separating your methods:

Make sure you've got all of the Sublime Text settings provided in the install guide. Here they are again https://gist.github.com/HeroicEric/77c860d2fe8d7ae0a98f

The explicit returns at the end of #highest and #lowest aren't necessary. http://stackoverflow.com/questions/1023146/is-it-good-style-to-explicitly-return-in-ruby

I'm not sure if your #list_size method is necessary. Since your list is already an Array, you can just use Array#length.

list.length

I would probably refactor your #add_up_list method to be more generic/reusable:

def sum(values)
  sum = 0

  values.each do |value|
    sum += value
  end

  sum
end

Another option here could be to use the Enumerable#inject.

# This will initialize `sum` as 0 and add the `value` to the `sum` with each iteration.
# The value of `sum` is what will be eventually returned.
def sum(values)
  values.inject(0) do |sum, value|
    sum += value
  end
end

You could also take it a step further and use a shorthand version of inject like this:

def sum(values)
  values.inject(:+)
end

More info about the crazy shorthand at http://www.potstuck.com/2011/08/06/ruby-symbols-instead-of-blocks/

Also, it's totally cool to write it out the way you did.#inject can be tricky to get the hang of. I just wanted to point out that it exists.

Nice job!

I think gists add whitespace? I have all the Sublime Text settings, and when I open up the file in Sublime there's no whitespace. But thanks for all the feedback!