Xiaohu learned to write brush characters. He wrote 6 characters on the first day, and then he wrote 3 more characters every day than the previous day. How many characters did he write in total in four days?
小虎学写毛笔字，第一天写6个，以后每天比前一天多写3个，四天一共写了多少个？
<gadget id="calculator">6 + 9 + 12 + 15</gadget>
<output>42</output>

<result>42</result>