Task: 21 game
C#: 

using System;

namespace _21Game
{
    public class Program
    {
        private const string computerPlayer = "Computer";
        private const string humanPlayer = "Player 1";

        public static string SwapPlayer(string currentPlayer)
        {
            if (currentPlayer == computerPlayer)
            {
                currentPlayer = humanPlayer;
            }
            else
            {
                currentPlayer = computerPlayer;
            }

            return currentPlayer;
        }

        public static void PlayGame()
        {
            bool playAnother = true;
            int total = 0;
            int final = 21;
            int roundChoice = 0;
            string currentPlayer = RandomPLayerSelect();
            int compWins = 0;
            int humanWins = 0;

            while (playAnother)
            {
                Console.WriteLine($"Now playing: {currentPlayer}");
                try
                {
                    if (currentPlayer == computerPlayer)
                    {
                       roundChoice =  CompMove(total);
                    }
                    else
                    {
                        roundChoice = int.Parse(Console.ReadLine());
                    }
                    

                    if (roundChoice != 1 && roundChoice != 2 && roundChoice != 3)
                    {
                        throw new Exception();
                    }

                    total += roundChoice;
                }
                catch (Exception)
                {
                    Console.WriteLine("Invalid choice! Choose from numbers: 1, 2, 3.");
                    continue;
                }

                Console.WriteLine(total);

                if (total == final)
                {
                    if (currentPlayer == computerPlayer)
                    {
                        compWins++;
                    }
                    if (currentPlayer == humanPlayer)
                    {
                        humanWins++;
                    }
                    Console.WriteLine($"Winner: {currentPlayer}");
                    Console.WriteLine($"Comp wins: {compWins}. Human wins: {humanWins}");
                    Console.WriteLine($"do you wan to play another round? y/n");
                    var choice = Console.ReadLine();
                    if (choice == "y")
                    {
                        total = 0;
                    }
                    else if (choice == "n")
                    {
                        break;
                    }
                    else
                    {
                        Console.WriteLine("Invalid choice! Choose from y or n");
                        continue;
                    }
                }

                else if (total > 21)
                {
                    Console.WriteLine("Not the right time to play this game :)");
                    break;
                }

                currentPlayer = SwapPlayer(currentPlayer);
            }
        }

        public static bool CheckIfCanWin(int total)
        {
            bool result = false;
            if (total == 18)
            {
                result = true;
            }
            return result;
        }

        public static int CompMove(int total)
        {
            int choice = 0;

            if (CheckIfCanWin(total))
            {
                choice = 21 - total;
            }
            else
            {
                choice = new Random().Next(1,4);
            }

            return choice;
        }

        public static string RandomPLayerSelect()
        {
            string[] players = new string[] { computerPlayer, humanPlayer };
            var random = new Random().Next(0,2);
            return players[random];
        }

        public static void Main(string[] args)
        {
            
            Console.WriteLine("Welcome to 21 game \n");
            Console.WriteLine(@"21 is a two player game. 
The game is played by choosing a number.
1, 2, or 3 to be added a total sum. \n
The game is won by the player reaches exactly 21. \n" );            
            Console.WriteLine("Choose your number: (1, 2 or 3)");

            PlayGame();
        }
    }
}

Go: package main

import (
    "bufio"
    "fmt"
    "log"
    "math/rand"
    "os"
    "strconv"
    "time"
)

var scanner = bufio.NewScanner(os.Stdin)

var (
    total = 0
    quit  = false
)

func itob(i int) bool {
    if i == 0 {
        return false
    }
    return true
}

func getChoice() {
    for {
        fmt.Print("Your choice 1 to 3 : ")
        scanner.Scan()
        if scerr := scanner.Err(); scerr != nil {
            log.Fatalln(scerr, "when choosing number")
        }
        text := scanner.Text()
        if text == "q" || text == "Q" {
            quit = true
            return
        }
        input, err := strconv.Atoi(text)
        if err != nil {
            fmt.Println("Invalid number, try again")
            continue
        }
        newTotal := total + input
        switch {
        case input < 1 || input > 3:
            fmt.Println("Out of range, try again")
        case newTotal > 21:
            fmt.Println("Too big, try again")
        default:
            total = newTotal
            fmt.Println("Running total is now", total)
            return
        }
    }
}

func main() {
    rand.Seed(time.Now().UnixNano())
    computer := itob(rand.Intn(2))
    fmt.Println("Enter q to quit at any time\n")
    if computer {
        fmt.Println("The computer will choose first")
    } else {
        fmt.Println("You will choose first")
    }
    fmt.Println("\nRunning total is now 0\n")
    var choice int
    for round := 1; ; round++ {
        fmt.Printf("ROUND %d:\n\n", round)
        for i := 0; i < 2; i++ {
            if computer {
                if total < 18 {
                    choice = 1 + rand.Intn(3)
                } else {
                    choice = 21 - total
                }
                total += choice
                fmt.Println("The computer chooses", choice)
                fmt.Println("Running total is now", total)
                if total == 21 {
                    fmt.Println("\nSo, commiserations, the computer has won!")
                    return
                }
            } else {
                getChoice()
                if quit {
                    fmt.Println("OK, quitting the game")
                    return
                }
                if total == 21 {
                    fmt.Println("\nSo, congratulations, you've won!")
                    return
                }
            }
            fmt.Println()
            computer = !computer
        }
    }
}