Use the three number cards 1, 4, and 7 to count according to the requirements. Choose one, two or three of the three cards, how many different natural numbers can be displayed?
用1、4、7三张数字卡片按要求摆数．从三张卡片中选一张，两张或者三张，可以摆出多少个不同的自然数？
<gadget id="calculator">3 + 6 + 6</gadget>
<output>15</output>

<result>15</result>