Follow the codes on this carefully.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace maths_quiz
{
    public partial class Form1 : Form
    {

        int numA;
        int numB;
        int totalA;
        int randMath;
        int check;

        public Form1()
        {
            InitializeComponent();
            playGame();
        }

        private void KeyisDown(object sender, KeyPressEventArgs e)
        {
            if (e.KeyChar == (char)Keys.P || e.KeyChar == 112)
            {
                check = 1;
                checkGame();
            }
            else if (e.KeyChar == (char)Keys.M || e.KeyChar == 109)
            {
                check = 2;
                checkGame();
            }
            else if (e.KeyChar == (char)Keys.X || e.KeyChar == 120)
            {
                check = 3;
                checkGame();
            }
            else if (e.KeyChar == (char)Keys.D || e.KeyChar == 100)
            {
                check = 4;
                checkGame();
            }
        }

        public void playGame()
        {
            Random ranNumber = new Random();
            numA = ranNumber.Next(1, 40);
            numB = ranNumber.Next(1, 50);
            randMath = ranNumber.Next(1, 5);
            symbol.Image = Properties.Resources.qq;


            switch (randMath)
            {
                case 1:
                    addNum();
                    break;
                case 2:
                    substractNum();
                    break;
                case 3:
                    multiplyNum();
                    break;
                case 4:
                    divideNum();
                    break;
            }


            num1.Text = Convert.ToString(numA);
            num2.Text = Convert.ToString(numB);
            total.Text = Convert.ToString(totalA);
        }

        public void addNum()
        {
            totalA = numA + numB;
        }
        public void substractNum()
        {
            totalA = numA - numB;
        }
        public void multiplyNum()
        {
            totalA = numA * numB;
        }
        public void divideNum()
        {
            totalA = numA / numB;
        }

        public void checkGame()
        {
            if (check == randMath)
            {
                switch (randMath)
                {
                    case 1:
                        symbol.Image = Properties.Resources.plus;
                        MessageBox.Show("Correct");
                        playGame();
                        break;
                    case 2:
                        symbol.Image = Properties.Resources.minus;
                        MessageBox.Show("Correct");
                        playGame();
                        break;
                    case 3:
                        symbol.Image = Properties.Resources.multi;
                        MessageBox.Show("Correct");
                        playGame();
                        break;
                    case 4:
                        symbol.Image = Properties.Resources.divide;
                        MessageBox.Show("Correct");
                        playGame();
                        break;
                }
            }
            else
            {
                MessageBox.Show("Wrong Answer");
            }
        }

    }
}