exit0

к примеру сложить первые три индекса

$arr = [0 => 12, 1 => 15, 2 => 15, 3 => 75, 4 => 77, 5 => 40, 6 => 15, 7 => 2, 8 => 40, 9 => 11, 10 => 8];

$slice = array_slice($arr, 0, 3);
$result = array_sum($slice) / sizeof($slice);

echo $result;