My contribution to the fray.

Update:Removed spurious hash left over from an earlier attempt.

#! perl -slw
use strict;

sub categorise {
    use POSIX qw[log10];
    my $n = shift;
    die 'Argument less than 1' unless $n > 0;

    return  5 if $n <= 5;
    return 10 if $n <=10;
    return 5 * int( $n / 5 + 1 ) if $n <= 100;

    my $scale = int( log10( $n ) );
    return (0+"1e$scale") * 0.25 
           * ( 1 + int( ( $n / (0+"1e$scale") ) / 0.25 ) );
}

my $t = 2.4;

for( 1 .. 20 ) {
    $t *= 1.55;
    printf "%5d : %5d\n", int($t), categorise( int( $t ) );
}

=pod comment Expose for a more thorough and randomised test.

printf "%5d : %5d\n", int($_), categorise( int( $_ ) )
    for 1..15,
    ( map{   10* ($_ + rand) } 2 .. 9 ),
    ( map{  100* ($_ + rand) } 1 .. 9 ),
    ( map{ 1000* ($_ + rand) } 1 .. 9 );
    
=cut
__END__
P:\>270920
    3 :     5
    5 :     5
    8 :    10
   13 :    15
   21 :    25
   33 :    35
   51 :    55
   79 :    80
  123 :   125
  192 :   200
  297 :   300
  461 :   475
  715 :   725
 1108 :  1250
 1718 :  1750
 2663 :  2750
 4129 :  4250
 6400 :  6500
 9920 : 10000
15376 : 17500
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