# Alien decimal numeral system

During our travels around the galaxy we found many alien cultures with different number systems. However, this specific civilization was quite interesting: they had a decimal numeral system, just like ours! To facilitate communications we learnt their decimal numerals and they learnt ours (0, 1, 2, etc...), along with the operators (+, -, ect...).

My alien friend showed me this...

(a) 2 + 2 = 4

...which was correct, as well as this:

(b) 1 + 5 = 6

And I thought it was all settled. But then they showed me this:

(c) 19 + 24 = 331

Which cannot be correct! Then, they came with:

(d) 28 * 17 = 2285

At that point I stopped them, saying "no, this is not right!". We sat down and they showed me the calculations step by step, and then I realised my mistaken assumption!

Now I can explain c and d. Can you?

Your assumption was that the aliens

whereas they

(c) 19 + 24 = 331

133 = 42 + 91

(d) 28 * 17 = 2285

5822 = 71 * 82

• Personally, I think they are right. But, I cannot say more as it would be a spoiler. Feb 14, 2022 at 12:27
• Nitpick: If they read right-to-left, wouldn't they have the operands on the right and the result on the left, e. g. 2285 = 28 * 17 ? Feb 14, 2022 at 13:56
• @Wastrel Why should aliens be expected to follow human conventions like that? Feb 14, 2022 at 16:05
• @Wastrel actually the aliens just read the numerals from right-to-left, not the operands. Amoz's answer is correct because that doesn't matter (if A = B then B = A), but the answer I had in my head was just 28 * 17 = 2285 ---> 82 * 71 = 5822, same order that we use. Feb 14, 2022 at 23:26