Aunt Zhang expresses a package of 5.9 kg to a friend from other places. The courier company stipulates: 15 yuan for 2 kg and less than 2 kg, and 4 yuan for each additional kg (if it is less than 1 kg, it will be calculated as 1 kg). How much does it cost Aunt Zhang to mail this package?
张阿姨给外地的朋友快递一个5.9千克的包裹，快递公司规定：2千克及2千克以内15元，每增加1千克加收4元(不满1千克的按1千克计算)．张阿姨邮寄这个包裹需花多少元？
<gadget id="calculator">4 * 4</gadget>
<output>16</output>

<gadget id="calculator">16 + 15</gadget>
<output>31</output>

<result>31</result>