Try this code:
using System;
class MarksRange
{
public static void Main()
{ int i, count80 = 0, count60 = 0, count40 = 0, count0 = 0;
float [] marks = {57.5F, 45.9F, 98.01F, 56.4F, 46.5F, 80, 82, 67,76,49, 91,55, 78,79, 19.5F, 25.8F, 35, 36,35,28,25.8F,46,55,59,68,97,85,48.5F,67,84};
for (i = 0; i<=29; i++)
{
If (marks[i] > 80 && marks [i] < 101)
{
count80 = count80 + 1;
}
else if(marks [i] > 60 && marks[i] < 81)
{
count60 = count60 + 1;
}
else if(marks [i] > 40 && marks[i] < 61)
{
count40 = count40 + 1;
}
else
{
count0 = count0 + 1;
}
}
Console.WriteLine("Students in the range of 81 - 100 : "+ count80);
Console.WriteLine("Students in the range of 61 - 80 : "+ count60);
Console.WriteLine("Students in the range of 41 - 60 : "+ count40);
Console.WriteLine("Students in the range of 0 - 40 : "+ count0);
Console.ReadLine();
}
}