Xiaohua practiced brush calligraphy. On the first day, he practiced 18 characters. On the second day, he practiced the same number of characters as the first day. On the third day, he practiced 15 characters. How many characters does he practice on average every day?
小华练毛笔字，第一天练了18个，第二天练的字数和第一天的字数相同，第三天练了15个，他平均每天练几个字？
<gadget id="calculator">18 + 18 + 15</gadget>
<output>51</output>

<gadget id="calculator">51 / 3</gadget>
<output>17</output>

<result>17</result>