var grade;
  
  if( marks > 90 )
    grade = 'AA';
  else if( marks > 80 )
    grade = 'AB';
  else if( marks > 70 )
    grade = 'BB';
  else if( marks > 60 )
    grade = 'BC';
  else if( marks > 50 )
    grade = 'CC';
  else if( marks > 40 )
    grade = 'CD';
  else if( marks > 30 )
    grade = 'DD';
  else
    grade = 'FF';
  
  console.log( grade );