BLACK JACK

[Introduction]

I have forgot its official name.

[How to play]

There need two persons to play the game [ex., persons (A) and (B)]. First, (A) says "1" or "1" "2". Secondly, (B) says a figure following the one (A) said last and may also say a figure following what (B) said just now. Thirdly, (A) says a figure following the one (B) said last and may also say a figure following what (A) said just now. Like this, (A) and (B) keep alternately saying the figures.
It is the loser that in the end has to say "21" while (A) and (B) keep alternately saying the figures. This is the only rule you have to know.

For example,
(A) : 1,2
(B) : 3
(A) : 4
(B) : 5,6
(A) : 7,8
(B) : 9
(A) : 10,11
(B) : 12,13
(A) : 14
(B) : 15
(A) : 16,17
(B) : 18
(A) : 19,20
(B) : 21
Then (A) is the winner.

[How to win]

I will explain the how-to reversing the order, that is, from "21" to "1".
In short, you should finally say "20".
For the purpose of your saying "20", you should make your opponent saying "18" or "19". For this, you must say "17".
For the purpose of your saying "17", you should make your opponent saying "15" or "16". For this, you must say "14".
If repeating this process, we can get the following figures: "20" "17" "14" "11" "8" "5" and "2". These figures run at intervals of "3".
Naturally your opponent might not know this how-to, and you will never fail to win by using this method.

[Postscript]

This rule may be changed as follows: You lose by saying, for example, "47" instead of "21" as well as saying, for example, five consecutive figures at once instead of two.
If applying such how-to, you will never fail to win.

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