20 children are playing a game of throwing handkerchiefs in a circle, what is the probability that the handkerchief falls behind each child?
20个小朋友围成一圈玩丢手绢的游戏，手绢落到每个小朋友后面的可能性都是多少．
<gadget id="calculator">20 - 1</gadget>
<output>19</output>

<gadget id="calculator">1 / 19</gadget>
<output>1/19 = around 0.052632</output>

<result>1/19 = around 0.052632</result>