<?php
    function CalculateAverage(array $data) {
        //return array with the firs item from every group and make sum from this array
        $sum = array_sum(array_map(function ($item) {
            return $item[0];
        }, $data));
        //temp variable for calculate the average
        $temp = 0;
        //itreate through the group
        for ($i = 0; $i < sizeof($data); ++$i) {
            $temp += ($data[$i][0] * $data[$i][1]);
        }
        //get the average
        return $temp / $sum;
    }
    function GetGroup($size, $total, $decrease, $stages) {
        $group = array();
        for ($i = 0; $i < $size - 1; ++$i) {
            array_push($group, array($decrease, $stages[$i]));
        }
        array_push($group, array($total - (($size - 1) * $decrease), $stages[$size - 1]));
        return $group;
    }
    $sales = 200;
    $stages = array(0.3, 0.35, 0.5, 0.75);
    $main_factor = 20;
    $max_stage = 3;

    $stages_params = array(0 => $stages[0]);
    for ($i = 1; $i <= $max_stage; ++$i) {
        array_push($stages_params, GetGroup($i + 1, $sales, $main_factor, $stages));
    }
    $stage_index = (floor($sales / $main_factor) > $max_stage) ? $max_stage : floor($sales / $main_factor);

    if (!isset($avg) && $stage_index !== 0) {
        $avg = CalculateAverage($stages_params[$stage_index]);
    }
    echo "<pre>";
    print_r( $stages_params);
    echo $avg;
?>