My solution is 'wrong' in some cases for 1 number. I have been testing this solution and its right! Its seems that the problem lays on the rounding used on the tests.

function calculate_scrap($scraps, $robots): int {
$tot = $robots * 50; // total iron needed, not the brute.

foreach ($scraps as $perc) {
$tot = ($tot * 100) / (100 - $perc);
}
return round($tot);
}