用GUI实现基本功能，界面上共显示20道加法练习(4行，每行5道)，用户可以单击按钮更换一批练习题。用户完成后提交答案,显示判题结果。

好的，这里是一个使用Python和Tkinter实现你要求的GUI程序的示例代码：

import random
import tkinter as tk

class MathQuiz:
    def __init__(self, master):
        self.master = master
        self.master.geometry("400x400")
        self.master.title("加法练习")

        # 初始化题目和答案
        self.numbers = []
        self.answers = []
        self.create_questions()

        # 创建题目和答案框架
        self.question_frame = tk.Frame(self.master)
        self.question_frame.pack()

        self.answer_frame = tk.Frame(self.master)
        self.answer_frame.pack()

        # 创建题目和答案控件
        self.create_question_widgets()
        self.create_answer_widgets()

        # 创建更换题目的按钮
        self.change_button = tk.Button(self.master, text="更换题目", command=self.change_questions)
        self.change_button.pack()

        # 创建提交答案的按钮
        self.submit_button = tk.Button(self.master, text="提交", command=self.check_answers)
        self.submit_button.pack()

    def create_questions(self):
        for i in range(20):
            a = random.randint(1, 10)
            b = random.randint(1, 10)
            self.numbers.append((a, b))
            self.answers.append(a + b)

    def create_question_widgets(self):
        for i in range(20):
            label = tk.Label(self.question_frame, text=f"{self.numbers[i][0]} + {self.numbers[i][1]} = ")
            label.grid(row=i // 5, column=i % 5)

    def create_answer_widgets(self):
        self.answer_entries = []
        for i in range(20):
            entry = tk.Entry(self.answer_frame)
            entry.grid(row=i // 5, column=i % 5)
            self.answer_entries.append(entry)

    def change_questions(self):
        # 重新生成题目和答案
        self.numbers = []
        self.answers = []
        self.create_questions()

        # 清空之前的题目和答案控件
        for widget in self.question_frame.winfo_children():
            widget.destroy()
        for widget in self.answer_frame.winfo_children():
            widget.destroy()

        # 重新创建题目和答案控件
        self.create_question_widgets()
        self.create_answer_widgets()

    def check_answers(self):
        result = []
        for i in range(20):
            answer = self.answer_entries[i].get()
            if answer == str(self.answers[i]):
                result.append("✓")
            else:
                result.append("✗")
        self.create_result_widgets(result)

    def create_result_widgets(self, result):
        for widget in self.answer_frame.winfo_children():
            widget.destroy()
        for i in range(20):
            label = tk.Label(self.answer_frame, text=result[i])
            label.grid(row=i // 5, column=i % 5)

root = tk.Tk()
app = MathQuiz(root)
root.mainloop()

这个程序实现了你要求的基本功能，使用了Python的Tkinter库来实现GUI界面。它首先生成了20道加法练习题，然后在界面上显示出来。用户可以在界面上输入答案，然后点击“提交”按钮进行判题。程序会将用户输入的答案与正确答案进行比较，然后在界面上显示出判题结果。同时，用户还可以点击“更换题目”按钮来更换一批新的练习题。