if ($number >= 10 ) { captures all numbers greater-or-equal than 10: 10, 11, 20, 50 etc.

When you enter 10 50, it is matched by if ($number >= 10 ) { but also by if ($number >= 20 ) { and also by if ($number >= 50 ) { and so on.

If I understand correctly you want to match ranges: 10-20 or 20-50. But not both. One way to achieve this is to use an elsif

if( $number >= 50 ){
  $sendthis = $number - 4;
} elsif( $number >= 20 ){
  $sendthis = $number - 3;
} elsif( $number >= 10 ){
  $sendthis = $number - 2;
}
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Notice that you start checking for the greatest number in the range (50) and go down. Also notice that indenting (append tab or spaces in code within code-blocks) makes reading code easier.

bw, bliako