php - figure out proper decremental values between a range -

i'm not quite sure how phrase question succinctly... also, i'm sure boils down basic math failure. anyways, goal count 1 20, assign decremented value count, starting 80 , ending in 2.

so basically,

$array = (   1 => 80,   ..   20 => 2 ); 

here code have come with, , seems close it's off, , can't figure out why :(

$array = array(); for( $x = 1; $x <= 20; $x++ ) {   $y = ( 80 - 2 ) / 20;   $p = ( 80 - ( $x * $y ) ) + ( $y / $x );   $array[$x] = $p; } echo "<pre>"; print_r($array); echo "</pre>"; 

this gives me:

array (     [1] => 80     [2] => 74.15     [3] => 69.6     [4] => 65.375     [5] => 61.28     [6] => 57.25     [7] => 53.2571428571     [8] => 49.2875     [9] => 45.3333333333     [10] => 41.39     [11] => 37.4545454545     [12] => 33.525     [13] => 29.6     [14] => 25.6785714286     [15] => 21.76     [16] => 17.84375     [17] => 13.9294117647     [18] => 10.0166666667     [19] => 6.10526315789     [20] => 2.195 ) 

could point out i'm going wrong or nudge me in right direction?

what want is:

$array = array(); ($x=1;$x<=20;$x++) {   $y = (80-2)/(20-1);   $p = 80-($x-1)*$y;   $array[$x] = $p; } 

explanation: start 80 , end 2. indexes go 1 20. means, have 20 elements, first , last fixed. so, have 20-1 = 19 "jumps" (or subtractions) until reach 2 80. each jump has same size. since total difference 80-2, each subtraction should have (80-2)/(20-1) = 78/19 size.

that's why should write

$p = 80-($x-1)*$y; 

note ($x-1) due fact subtraction starts on second element (you want start 80).