Dad filled a sprayer with 8 liters of water, he was supposed to add 32 pills, but he only added 16 pills. After using up two liters of the solution, he discovered the mistake, so he added another two liters of water, and then added enough potion to meet the requirements. How many pills should he add.
爸爸在一个喷雾器内装入8公升的水，他本应加入32颗药剂，但他却只加入16颗．当用掉二公升溶液后，他才发现这个错误，于是他再加入二公升的水，并再加入足够数量的药剂以符合要求．他应再加入多少颗药剂．
<gadget id="calculator">32 - 16</gadget>
<output>16</output>

<gadget id="calculator">16 + 4</gadget>
<output>20</output>

<result>20</result>