For #12 it's a math thing. We count by the DEC or decimal system which is base-10, which every time we reach 10, we write ten so 25 is still 25. If we count by OCT or the octal system, we are counting every 8 that we meet. It looks like this: 1 2 3 4 5 6 7 10. By translating 31 in the octal system into a number in the decimal system, we get 25.
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