| If the four back pins are labeled A,B,C,D, then we have:
A B C D
A+B B+C C+D
A+2B+C B+2C+D
A+3B+3C+D
(reduced mod 10). Since the labels are (in some order) the
integers 0 through 9, their sum is 45 (mod 10). So we get an
equation:
4A + 9B + 9C + 4D = 5(mod 10)
The pin labeled 0 must be in front, since otherwise it would cause
equal labels to be assigned to the pin to its side and the one in
front of and between them. So we have a second equation:
A + 3B + 3C + D = 0(mod 10)
Combining, we find
D = 5-A (mod 10)
C = 5-B (mod 10).
So our setup is
A B 5-B 5-A
A+B 5 -A-B
A+B+5 5-A-B
0
with everything being considered mod 10.
The opponent just bowled another strike. We'd better hurry up.
Now A is neither 0 nor 5, so -A is not equal to A. Where can the
pin labeled -A go? Not in the first row (the lone pin there is
already labeled 0). Not in the second row, since the two pins
there add to 0 (or 10), so if one was -A the other would be A. Not
in the third row, because having (-A) next to (5) would cause the
pin in front of them to be labeled (5-A), duplicating one in the
back row.
So the (-A) goes in the back row: either B=-A or 5-B=-A. Now, if
B=-A then the left-hand pin of the third row is 0, duplicating the
head pin. The only possibility is that 5-B=-A, which means
B=5+A, so that the setup is:
A 5+A -A -5-A
5+2A 5 5-2A
2A -2A
0
Furthermore this will work with any A except 5 or 0 (which would
duplicate an existing label). For example with A=4 we get
4 9 6 1
3 5 7
8 2
0
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