Among the 30 natural numbers 1, 2, 3, ... 30, how many numbers can be taken at most, so that the sum of any two different numbers among the taken out numbers is not a multiple of 9.
在1，2，3，…30，这30个自然数中，最多能取出多少个数，使取出的这些数中任意两个不同的数的和都不是9的倍数．
<gadget id="calculator">8 / 8</gadget>
<output>1</output>

<gadget id="calculator">1 + 4 + 4 + 4 + 3</gadget>
<output>16</output>

<result>16</result>