How to score 100 in ICSE Computer Applications: stop the leaks.

import java.util.Scanner;
class Marks {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String n = sc.nextLine();
        int a = sc.nextInt(), b = sc.nextInt(), c = sc.nextInt();
        double x = (a + b + c) / 3;
        System.out.println(n);
        System.out.println(x);
        System.out.println(x >= 80 ? 'A' : 'B');
    }
}

Wrong class name. No input(), compute() or display(). No data members. Integer division. Names a, b, c and x describe nothing. Output has no labels.

import java.util.Scanner;

class Student {
    String name;      // student's name
    int m1, m2, m3;   // marks in three tests
    double avg;       // average of the three tests
    char grade;       // 'A' if avg >= 80, else 'B'

    void input() {
        Scanner sc = new Scanner(System.in);
        name = sc.nextLine();
        m1 = sc.nextInt();
        m2 = sc.nextInt();
        m3 = sc.nextInt();
    }

    void compute() {
        avg = (m1 + m2 + m3) / 3.0;
        grade = (avg >= 80) ? 'A' : 'B';
    }

    void display() {
        System.out.println("Name: " + name);
        System.out.println("Average: " + avg);
        System.out.println("Grade: " + grade);
    }

    public static void main(String[] args) {
        Student ob = new Student();
        ob.input();
        ob.compute();
        ob.display();
    }
}

// Input: Riya Sen / 78 85 84
// Output:
// Name: Riya Sen
// Average: 82.33333333333333
// Grade: A

int c = 0;
for (int i = 2; i < n; i++)
    if (n % i == 0)
        c++;
if (c == 0)
    System.out.println("Prime");
else
    System.out.println("Not prime");

// n = 7: Prime
// n = 9: Not prime
// n = 1: Prime     (wrong)

int factors = 0;            // divisors from 1 to n
for (int i = 1; i <= n; i++)
    if (n % i == 0)
        factors++;
if (factors == 2)           // exactly 1 and n
    System.out.println(n + " is a prime number");
else
    System.out.println(n + " is not a prime number");

// n = 7: 7 is a prime number
// n = 9: 9 is not a prime number
// n = 1: 1 is not a prime number

String ans = sc.next();
if (ans == "yes")
    System.out.println("== says equal");
else
    System.out.println("== says not equal");
if (ans.equals("yes"))
    System.out.println("equals says equal");

// Output:
// == says not equal
// equals says equal

== asks whether two names point to the same String object; a word read by Scanner is a new object, so == is false even when the letters match.