Students in Class 3 and 1 buy colored paper to decorate the classroom. There are 15 sheets of red paper, which is 3 times less than the blue paper. How many sheets of paper of these two different colors are there in total?
三1班同学买彩纸布置教室，红色的有15张，比蓝色的3倍少3张，这两种不同颜色的纸一共有多少张．
<gadget id="calculator">15 + 3</gadget>
<output>18</output>

<gadget id="calculator">18 / 3</gadget>
<output>6</output>

<gadget id="calculator">6 + 15</gadget>
<output>21</output>

<result>21</result>