Write the numbers from 1 to 21 on 21 number cards, randomly draw one of the random cards, what is the probability of drawing a multiple of 3?
21张数字卡片上分别写上1～21各数，打乱卡片任意抽一张，抽到3的倍数的可能性是多少．
<gadget id="calculator">7 / 21</gadget>
<output>1/3 = around 0.333333</output>

<result>1/3 = around 0.333333</result>