Well it is kind a possible with arrays, but the simplest way to do this is:
program prog_1;
var number:integer;
character:char;
begin
write('Enter the number : ');
readln(number);
if (number>0) and (number<24) then character:='E'
else if (number>25) and (number<49) then character:='D'
else if (number>50) and (number<70) then character:='C'
else if (number>71) and (number<90) then character:='B'
else if (number>91) and (number<100) then character:='A'
else writeln('UNDEFINED!');
writeln(character);
end.
In this way u get what u wanted.
Maybe there is a easier way, but i just cant think about anything simpler than this one.
Sorry for bad english.