Xiao Ming has 21 cards, and after giving Xiao Hong 3 cards, they both have the same number. How many cards did Xiaohong originally have?
小明有21张卡片，给小红3张后两人一样多．小红原来有多少张卡片？
<gadget id="calculator">21 - 3 - 3</gadget>
<output>15</output>

<result>15</result>