HW I got help from @Khulud-Alsharqawi

src/main/kotlin/Main.kt

@@ -2,5 +2,105 @@ fun main(args: Array<String>) {
 
     // write your code here
 
+    val course1 = Course("Korean language")
+    course1.number_of_lectures = 5
+    val prof = Professor("kim", "heechul", "1234567890")
+    if (course1.assignProfessor(prof))
+        println("professor added successfully")
+    prof.courses.add("Korean language")
+    if (course1.students.add(Student("jun", "ki")))
+        println("the student added successfully")
+    println("size ${course1.students.size}")
+    println("course information:${course1.courseInfo()}")
 
-}
+
+
+}
+class Course(var courseName: String) {
+    var number_of_lectures: Int = 0
+    var students = mutableListOf<Student>()
+    var professor: Professor = Professor()
+    var isTaken = false
+
+    fun numberOfStudents(): Int {
+        return students.size
+    }
+
+    fun assignProfessor(p: Professor): Boolean {
+        return if (!isTaken && p.firstName != professor.firstName) {
+            this.professor = p
+            this.professor.courses.add(this.courseName)
+            this.isTaken = true
+            true
+        } else {
+            println("Can't assign the professor ${p.fullName()}, there is anthor Professor ${this.professor.fullName()}!")
+            false        }
+    }
+
+    fun professorName(): String {
+        return this.professor.fullName()
+    }
+
+    fun enroll(s: Student): Boolean {
+        return if (this.students.size <= 20 && !this.students.contains(s)) {
+            this.students.add(s)
+            s.courses.add(this.courseName)
+            true
+        } else {
+            println("Student can't be enrolled")
+            false
+        }
+    }
+
+    fun courseInfo(): String {
+        return "Name of course: ${this.courseName}, Number of lectures: ${this.number_of_lectures}, Name of professor: " +
+                "${professor.fullName()} \n,Number of the students: ${numberOfStudents()},Name of students:${printStudentList()}"
+    }
+
+    fun printStudentList(): String {
+        var studentName = ""
+        for (i in 0 until students.size - 1) {
+            studentName += "${i + 1}-${students[i].fullName()} ,"
+        }
+        studentName += "${students.size}-${students[students.size - 1].fullName()}"
+        return studentName
+    }
+}
+class Student() {
+    var firstName =""
+    var lastName =""
+    var courses = mutableListOf<String>()
+    constructor( firstName: String,  lastName: String):this(){
+        this.firstName=firstName
+        this.lastName=lastName
+
+    }
+    fun fullName(): String {
+        return "${firstName} \t ${lastName}"
+    }
+    fun addcourses (coursesName:String){
+        courses.add(coursesName)
+    }
+    fun deleteSubject (coursesName:String){
+        courses.remove(coursesName)
+    }
+}
+class Professor( ) {
+
+    var firstName =""
+    var lastName =""
+    var telephone = ""
+        set(value) {
+            if (value.length == 10) field = value else println("Enter a valid number")
+        }
+    var courses = mutableListOf<String>()
+    constructor(firstName: String, lastName: String, telephone: String): this() {
+        this.firstName = firstName
+        this.lastName = lastName
+        this.telephone = telephone
+    }
+    fun fullName(): String {
+        return "${firstName} \t ${lastName}"
+    }
+
+}

src/main/kotlin/Professor.kt

@@ -0,0 +1,19 @@
+class Professor( ) {
+
+   var firstName =""
+   var lastName =""
+    var telephone = ""
+    set(value) {
+        if (value.length == 10) field = value else println("Enter a valid number")
+    }
+    var courses = mutableListOf<String>()
+    constructor(firstName: String, lastName: String, telephone: String): this() {
+        this.firstName = firstName
+        this.lastName = lastName
+        this.telephone = telephone
+    }
+    fun fullName(firstName : String ,lastName :String): String {
+        return "${firstName} \t ${lastName}"
+    }
+
+}

src/main/kotlin/Student.kt

@@ -0,0 +1,19 @@
+class Student() {
+    var firstName =""
+    var lastName =""
+    var courses = mutableListOf<String>()
+    constructor( firstName: String,  lastName: String):this(){
+        this.firstName=firstName
+        this.lastName=lastName
+
+    }
+    fun fullName(firstName : String ,lastName :String): String {
+        return "${firstName} \t ${lastName}"
+    }
+    fun addcourses (coursesName:String){
+        courses.add(coursesName)
+    }
+    fun deleteSubject (coursesName:String){
+        courses.remove(coursesName)
+    }
+}