Some tricky parts were included in this exam exercise. For this solution, the crucial point was to first go trough the rarities/sortedPlants collection and to re-assign two new values to the ratings collection for each plant : the rarity number of the plant and the calculated average of the plant (line 77 - 92). In addition, while the code is still processing the “Exhibition” loop, an additional validation had to be included for non-existing plant-names (line 42 – 46).

Best,

New code(100%) :

using System;
using System.Linq;
using System.Collections.Generic;

namespace Plant_Discovery
{
    class Program
    {
        static void Main(string[] args)
        {
            int numberOfPlants = int.Parse(Console.ReadLine());
            Dictionary<string, int> rarities = new Dictionary<string, int>();
            Dictionary<string, List<double>> ratings = new Dictionary<string, List<double>>();

            for (int i = 0; i < numberOfPlants; i++)
            {
                string[] informtionForPlants = Console.ReadLine()
                    .Split("<->");
                string plant = informtionForPlants[0];
                int rarity = int.Parse(informtionForPlants[1]);

                if (rarities.ContainsKey(plant))
                {
                    rarities[plant] = rarity;
                }
                else
                {
                    rarities.Add(plant, rarity);
                    ratings.Add(plant, new List<double>());
                }
            }

            string command;

            while ((command = Console.ReadLine()) != "Exhibition")
            {
                string[] receivedCommand = command.Split(": ");
                string firstPartOfCommand = receivedCommand[0];
                string[] secondPartOfCommand = receivedCommand[1].Split(" - ");
                string plant = secondPartOfCommand[0];

                if (!ratings.ContainsKey(plant))
                {
                    Console.WriteLine("error");
                    continue;
                }

                if (firstPartOfCommand == "Rate")
                {
                    int rating = int.Parse(secondPartOfCommand[1]);

                    ratings[plant].Add(rating);
                }
                else if (firstPartOfCommand == "Update")
                {
                    int rarity = int.Parse(secondPartOfCommand[1]);

                    rarities[plant] = rarity;
                }
                else if (firstPartOfCommand == "Reset")
                {
                    //ratings[plant].RemoveAll(x => x > 0);
                    ratings[plant].Clear();
                }
                else
                {
                    Console.WriteLine("error");
                }
            }

            Console.WriteLine("Plants for the exhibition:");

            var sortedPlants = rarities
                .OrderByDescending(p => p.Value)
                .ToDictionary(key => key.Key, value => value.Value);

            foreach (var (plant, rarity) in sortedPlants)
            {
                if (ratings.ContainsKey(plant))
                {
                    double average = 0;

                    if (ratings[plant].Any())
                    {
                        average = ratings[plant].Average();
                    }

                    ratings[plant].Clear();
                    ratings[plant].Add(rarity);
                    ratings[plant].Add(average);
                }
            }

            foreach (var (plant, data) in ratings.OrderByDescending(p => p.Value[0]).ThenByDescending(p => p.Value[1]))
            {

                Console.WriteLine($"- {plant}; Rarity: {data[0]}; Rating: {data[1]:f2}");

                /*foreach (var item in ratings.OrderByDescending(x => x.Value.Sum() / x.Value.Count))
                {
 
                }*/
            }
        }
    }
}