Task: 21 game
C++: 

#include <cstdlib>
#include <ctime>
#include <iostream>
#include <iomanip>
#include <limits>

using namespace std;
#define _(STR) STR


class Model
{
public:
    static const int GOAL = 21;
    static const int NUMBER_OF_PLAYERS = 2;
    static const int MIN_MOVE = 1;
    static const int MAX_MOVE = 3;
    int bestMove();
    bool update(const char* player, int move);
    bool isGameBegin();
    bool isGameOver();
protected:
    friend class View;
    View* view = nullptr;
    const char* player = nullptr;
    int oldTotal = 0;
    int newTotal = 0;
    int lastMove = 0;
};


class View
{
protected:
    Model* model;
public:
    View(Model* model);
    void init(const char* player);
    void update();
};


class Controller
{
protected:
    Model* model;
    View* view;
public:
    Controller(Model* model, View* view);
    int input();
    void clear();
    void run();
};


int Model::bestMove()
{
    
    

    int move = MIN_MOVE;
    for (int i = MIN_MOVE; i <= MAX_MOVE; i++)
        if ((newTotal + i - 1) % (MAX_MOVE + 1) == 0)
            move = i;
    for (int i = MIN_MOVE; i <= MAX_MOVE; i++)
        if (newTotal + i == GOAL)
            move = i;
    return move;
}

bool Model::update(const char* player, int move)
{
    if (move >= MIN_MOVE && move <= MAX_MOVE && newTotal + move <= GOAL)
    {
        this->player = player;
        oldTotal = newTotal;
        newTotal = oldTotal + move;
        lastMove = move;
        view->update();
        return true;
    }
    else
        return false;
}

bool Model::isGameBegin()
{
    return oldTotal == 0;
}

bool Model::isGameOver()
{
    return newTotal == GOAL;
}


View::View(Model* model)
{
    this->model = model;
    model->view = this;
}

void View::init(const char* player)
{
    if (model->newTotal == 0)
        cout << _("----NEW GAME----\n\n")
             << _("The running total is currently zero.\n")
             << _("The first move is ") << player << _(" move.\n\n");
}

void View::update()
{
    cout << setw(8) << model->player << ": " << model->newTotal << " = "
         << model->oldTotal << " + " << model->lastMove << endl << endl;
    if (model->isGameOver())
        cout << endl << _("The winner is ") << model->player << _(".\n\n\n");
}


Controller::Controller(Model* model, View* view)
{
    this->model = model;
    this->view = view;
}

void Controller::run()
{
    if (rand() % Model::NUMBER_OF_PLAYERS == 0)
    {
        view->init("AI");
        model->update("AI", model->bestMove());
    }
    else
        view->init("human");

    while (!model->isGameOver())
    {
        while (!model->update("human", input()))
            clear();
        model->update("AI", model->bestMove());
    }
}

int Controller::input()
{
    int value;
    cout << _("enter a valid number to play (or enter 0 to exit game): ");
    cin >> value;
    cout << endl;
    if (!cin.fail())
    {
        if (value == 0)
            exit(EXIT_SUCCESS);
        else
            return value;
    }
    else
        return model->MIN_MOVE - 1;
}

void Controller::clear()
{
    cout << _("Your answer is not a valid choice.") << endl;
    cin.clear();
    cin.ignore((streamsize)numeric_limits<int>::max, '\n');
}


int main(int argc, char* argv)
{
    srand(time(NULL));

    cout << _(
        "21 Game                                                          \n"
        "                                                                 \n"
        "21 is a two player game, the game is played by choosing a number \n"
        "(1, 2, or 3) to be added to the running total. The game is won by\n"
        "the player whose chosen number causes the running total to reach \n"
        "exactly 21. The running total starts at zero.                    \n\n");

    while (true)
    {
        Model* model = new Model();
        View* view = new View(model);
        Controller* controler = new  Controller(model, view);

        controler->run();

        delete controler;
        delete model;
        delete view;
    }
    return EXIT_SUCCESS; 
}

VB: 

Class MainWindow
    Private Const GOAL As Integer = 21
    Private total As Integer = 0
    Private random As New Random

    Private Sub Update(box As TextBox, player As String, move As Integer)
        total = total + move
        box.Text = move
        boxTotal.Text = total
        If total + 1 > GOAL Then button1.IsEnabled = False
        If total + 2 > GOAL Then button2.IsEnabled = False
        If total + 3 > GOAL Then button3.IsEnabled = False
        If total = GOAL Then
            winner.Content = $"The winner is {player}."
        End If
    End Sub

    Private Sub Ai()
        Dim move As Integer = 1
        For i = 1 To 3
            If (total + i - 1) Mod 4 = 0 Then move = i
        Next i
        For i = 1 To 3
            If total + i = GOAL Then move = i
        Next i
        Update(boxAI, "AI", move)
    End Sub

    Private Sub Choice(sender As Object, e As RoutedEventArgs) _
            Handles button1.Click, button2.Click, button3.Click
        Update(boxHuman, "human", sender.Content)
        If total < GOAL Then Ai()
    End Sub

    
    
    
    Private Sub StartGame(sender As Object, e As RoutedEventArgs) Handles restart.Click

        total = 0

        boxAI.Text = ""
        boxHuman.Text = ""
        boxTotal.Text = ""
        
        winner.Content = ""

        button1.IsEnabled = True
        button2.IsEnabled = True
        button3.IsEnabled = True

        
        
        
        If random.Next(2) = 0 Then
            first.Content = "First player is AI player."
            Ai()
        Else
            first.Content = "First player is human player."
        End If
    End Sub
End Class