function getGrade(score) {
    let grade;
    if(score >= 0 && score <= 5) grade = 'F';
    else if(score > 5 && score <= 10) grade = 'E';
    else if(score > 10 && score <= 15) grade = 'D';
    else if(score > 15 && score <= 20) grade = 'C';
    else if(score > 20 && score <= 25) grade = 'B';
    else if(score > 25 && score <= 30) grade = 'A';
    return grade;
}