Feature/grade added

Calculator.java

@@ -0,0 +1,9 @@
+public class Calculator {
+    public int add(int a, int b) { return a + b; }   // resolved merge conflict
+    public int subtract(int a, int b) { return a - b; }
+    public int multiply(int a, int b) { return a * b; }
+    public int divide(int a, int b) {
+        if (b == 0) throw new IllegalArgumentException("Cannot divide by zero");
+        return a / b;
+    }
+}

Grade.java

@@ -0,0 +1,8 @@
+public class Grade {
+    public String getGrade(int marks) {
+        if (marks >= 90) return "A";
+        if (marks >= 75) return "B";
+        if (marks >= 60) return "C";
+        return "F";
+    }
+}

Student.java

@@ -0,0 +1,12 @@
+public class Student {
+    private String name;
+    private int id;
+
+    public Student(String name, int id) { this.name = name; this.id = id; }
+
+    public String getName() { return name; }
+    public int getId() { return id; }
+
+    public void setName(String name) { this.name = name; }
+    public void setId(int id) { this.id = id; }
+}