可以将母羊按年龄分为五种,之后统计每种的母羊的数量最后相加就可以了
<?php
$one_year = 1;
$two_year = 0;
$three_year = 0;
$four_year = 0;
$five_year = 0;
for ($i = 1; $i <= 20; ++$i){
$temp = $two_year + $four_year;
$five_year = $four_year;
$four_year = $three_year;
$three_year = $two_year;
$two_year = $one_year;
$one_year = $temp;
}
$sum = $one_year+$two_year+$four_year+$three_year+$five_year;