Assuming $top is supposed to be at the top of the page and $bottom is supposed to be at the bottom of the page and pages start at one and the first image index you want is zero, then you want: 

$top    = $page * 20 - 20; 
$bottom = $top + 19;

[download]
If pages and image indexes both start at zero, you want: 
$top    = $page * 20; 
$bottom = $top + 19;

[download]
If image indexes start at one and pages start at one, you want: 
$top    = $page * 20 - 19; 
$bottom = $top + 19; 

# A clearer Alternative: 
$bottom = $page * 20; 
$top    = $bottom - 19;

[download]
If image indexes start at one and pages start at zero, you want: 
$top    = $pages * 20 + 1; 
$bottom = $top + 19;

[download]
This stuff really isn't that difficult. I can't imagine why everyone has had so much trouble with it.

I assume, BTW, that you are using Tie::IxHash or something. Otherwise you absolutely cannot rely on the order of the keys.

-sauoq
"My two cents aren't worth a dime.";

In reply to Re: mathematical equation by sauoq
in thread mathematical equation by Anonymous Monk

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