When Rivka names what her number means as does Chaya in chapter 15.
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It might mean that might your a jew 1 might stand for everyone she cared about died 6 is probably her family and 5 anf 4 are her family dieing one at a time.
The devils arithmetic
Rivka's explanation can be found on page 113; the explanation for the number above is Hannah's not Rivka's.
Rivka's number is J18202; J, because she's a Jew, 1 for herself because she's alone, 8 is for her family because they were 8 in the village, 2 because that is all that's left of the family (Rivka and Wolfe), 0 because that is what Wolfe believes himself to be, and 2 because when everything is over they will be 2 again.
The Devil's Arithmetic/ Page 113