An alternative (and more logical since there is no reason to single out the 5 but keep the 9 and 6 as they are) would be that all number symbols stand for one less than they do in our normal system.
Translating to regular decimal:
If 8 is twice 4; what is 5 times 4 in the same notation?
The answer to that is of course 20, but we need to translate that back into the shifted notation, so we get 31. (Asking for 8 times 8 would have been interesting - it would mean 7 by 7, so 49, which would have to cycle around to 50.)
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