完善

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

namespace 简易运算
{
    class Pack   //封装
    {
        private int a;
        private int b;
        private string oper;
        private int anster = 0;

        public int OP
        {
            set
            {
                a = value;
            }
            get
            {
                return a;
            }
        }
        public int Pt
        {
            set
            {
                b = value;
            }
            get
            {
                return b;
            }
        }
        public string Oper
        {
            set
            {
                oper = value;

            }
            get
            {
                return oper;
            }
        }
        public int Aster
        {
            get
            {
                return anster;
            }
        }
        public int yusuan()
        {
            switch (oper)
            {
                case "+":
                    anster = a + b;
                    break;
                case "-":
                    if (a > b)
                        anster = a - b;
                    //else
                    //    throw new Exception("被减数不能小于零！");
                    break;
                case "*":
                    anster = a * b;
                    break;
                case "/":
                    //if (b == 0)
                    //{
                    //    throw new Exception("被除数不能小于零！");

                    //}
                    //else
                    anster = a / b;
                    break;
            }
            return anster;

        }
        public int fanhui()
        {
            return anster;
        }

    }
}

Form1代码:

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;
using System.IO;

namespace 简易运算
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        string path = "E:/";
        public static int count = 0;
        public static int t = 0;
        public static int right = 0;
        public static int sum;
        int n = 0;
        int m = 0;
        private void Form1_Load(object sender, EventArgs e)
        {
            Class1 s = new Class1();
            s.text1();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            StreamWriter l1 = File.AppendText("1.txt");
            l1.WriteLine(text1.Text);
            l1.Close();
            StreamWriter l2 = File.AppendText("2.txt");
            l2.WriteLine(comboBox1.SelectedItem.ToString());
            l2.Close();
            StreamWriter l3 = File.AppendText("3.txt");
            l3.WriteLine(text2.Text);
            l3.Close();
            richTextBox1.Text += text1.Text + comboBox1.SelectedItem.ToString() + text2.Text + "
";
            n++;
            text1.Text = "";
            text2.Text = "";
            savet.Enabled = true;
            opent.Enabled = true;
            text1.Focus();
        }

        private void savet_Click(object sender, EventArgs e)
        {
            SaveFileDialog TxtSaveDialog = new SaveFileDialog();
            TxtSaveDialog.Filter = "RTF文件(*.RTF)|*.RTF";
            if (File.Exists(path))
            {
                this.richTextBox1.SaveFile(path, RichTextBoxStreamType.RichText);
                MessageBox.Show("保存成功", "提示信息", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);
                this.richTextBox1.Clear();
                savet.Enabled = false;
            }
            else
            {
                if (TxtSaveDialog.ShowDialog() == DialogResult.OK)
                {

                    this.richTextBox1.SaveFile(TxtSaveDialog.FileName, RichTextBoxStreamType.RichText);
                    MessageBox.Show("保存成功", "提示信息", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);
                    this.richTextBox1.Clear();
                    savet.Enabled = false;
                }
            }
        }

        private void opent_Click(object sender, EventArgs e)
        {
            OpenFileDialog TxTOpenDialog = new OpenFileDialog();
            TxTOpenDialog.Filter = "RTF文件(*.RTF)|*.RTF";
            if (TxTOpenDialog.ShowDialog() == DialogResult.OK)
            {
                path = TxTOpenDialog.FileName;
                this.richTextBox1.LoadFile(TxTOpenDialog.FileName, RichTextBoxStreamType.RichText);
                savet.Enabled = false;
                opent.Enabled = false;
                MessageBox.Show("打开成功", "提示信息", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);
            }
        }

        private void button2_Click(object sender, EventArgs e)
        {
            jishi.Text = t.ToString();
            timer1.Enabled = true;
            timer1.Interval = 1000;
            timer1.Start();
            string[] l1 = new string[100];
            l1 = File.ReadAllLines("1.txt");
            string[] l2 = new string[100];
            l2 = File.ReadAllLines("2.txt");
            string[] l3 = new string[100];
            l3 = File.ReadAllLines("3.txt");
            Box1.Text = l1[0];
            Box4.Text = l2[0];
            Box2.Text = l3[0];
        }

        private void Box3_KeyDown(object sender, KeyEventArgs e)
        {
            //string s = Box4.Text;
            Pack operast = new Pack();
            operast.OP = int.Parse(Box1.Text);


            operast.Pt = int.Parse(Box2.Text);

            operast.Oper = Box4.Text;
            //switch (s)
            //{
            //    case "+":
            //        sum = int.Parse(Box1.Text) + int.Parse(Box2.Text);

            //        break;
            //    case "-":
            //        sum = int.Parse(Box1.Text) - int.Parse(Box2.Text);

            //        break;
            //    case "*":
            //        sum = int.Parse(Box1.Text) * int.Parse(Box2.Text);

            //        break;
            //    case "/":
            //        sum = int.Parse(Box1.Text) / int.Parse(Box2.Text);

            //        break;
            //    default:
            //        break;
            //}


            operast.yusuan();
            if (e.KeyCode == Keys.Enter)
            {
                if (operast.Aster.ToString()==Box3.Text  )
                {
                    MessageBox.Show("回答正确哦！");
                    count++;
                    right++;
                }
                else
                {
                    MessageBox.Show("回答错误哦！");
                    count++;
                }
                m++;
                if (m < n)
                {
                    Box3.Clear();
                    string[] ll = new string[100];
                    ll = File.ReadAllLines("1.txt");
                    Box1.Text = ll[m];
                    string[] l2 = new string[100];
                    l2 = File.ReadAllLines("2.txt");
                    Box4.Text = l2[m];
                    string[] l3 = new string[100];
                    l3 = File.ReadAllLines("3.txt");
                    Box2.Text = l3[m];
                }
                else
                {
                    jishi.Enabled = false;
                    Form2 frm = new Form2();
                    frm.ShowDialog();
                }
            }
        }

        private void timer1_Tick(object sender, EventArgs e)
        {
            t = t + 1;
            jishi.Text = t.ToString();
        }
    }
}

Form2代码：
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 简易运算
{
    public partial class Form2 : Form
    {
        public Form2()
        {
            InitializeComponent();
        }

        private void Form2_Load(object sender, EventArgs e)
        {
            textBox2.Text = Form1.count.ToString();
            textBox1.Text = Form1.right.ToString();
            textBox3.Text = ((Form1.right / (double)(Form1.count)) * 100).ToString() + "%";
        }

        private void label1_Click(object sender, EventArgs e)
        {

        }
    }
}