https://www.acmicpc.net/problem/9498
if else문을 사용한다.
import java.util.*;
public class Main{
public static void main(String args[]){
Scanner sc = new Scanner(System.in);
int score = sc.nextInt();
if(score >= 90 && score <= 100)
System.out.println("A");
else if(score >= 80 && score <= 89)
System.out.println("B");
else if(score >= 70 && score <= 79)
System.out.println("C");
else if(score >= 60 && score <= 69)
System.out.println("D");
else
System.out.println("F");
}
}