Ruby solution for the task mentioned in the article http://grishaev.me/why-clj

middle_rate.rb

class MiddleRate
  attr_reader :data

  def initialize(data)
    @data = data
  end

  def call
    # age rates got { 18 => [30, 15], 50 => [35] }
    age_rates = data.each_with_object(memory_hash) do |data_item, acc|
      acc[data_item[:age]].push(data_item[:rate])
    end

    age_rates.reduce({}) do |acc, (age, rates)|
      acc.merge(age => rates.reduce(:+).to_f / rates.count)
    end
  end

  private

  def memory_hash
    Hash.new { |hash, key| hash[key] = [] }
  end
end

middle_rate_spec.rb

require './middle_rate.rb'

RSpec.describe MiddleRate do
  subject(:middle_rate) { described_class.new(data) }

  let(:data) do
    [{ age: 18, rate: 30 },
     { age: 18, rate: 15 },
     { age: 50, rate: 35 }]
  end

  let(:expected_output) do
    { 18 => 22.5, 50 => 35.0 }
  end

  it 'computes the middle rate' do
    expect(middle_rate.call).to eq expected_output
  end
end