<?php

function f($x){
  return 20/pow(1+$x,14/365)+24/pow(1+$x,28/365)+1-40;
}

$min=-999999999;
$max=999999999;
while(abs($max-$min)>0.000001){
  $cur=($max+$min)/2;
  $rez=f($cur);
  if ($rez<0) $max=$cur; else $min=$cur;
}

echo $cur*100;

675.15619694078