I think this might do it, if it doesnÂ´t, I would have found 2 different answers for this one Sayumi, haha
HereÂ´s my answer,First I tried to think the relationship between the numbers in the middle with the one at their side. So, 11235 (9) 121416,
if you lin this two and you sum, you get 233766, this made me think:" One number, two 3Â´s, one number, two 6Â´s, so maybe one number and two 9Â´s. 9 is the middle number.
After this 2468 (5) 36912,
you also line up and you sum the digits: 2+3, 4+6, 6+9, 8+12 and you get 5,10,15,20, multiples of 5 in order. 5 is the midle number.
Finally 000 (4) 888,
4x222=888. 4 is the middle number.
This cost me a lot, a relationship between 876 and 19232931, WTF!!! i made the same as the others, line up and sum, but ... terible. After that I continued with the general idea, and suddenly I thought: "Maybe I have to sum all the digits: 8+7+6+1+9+2+3+2+9+3+1= 51
51=17x3 -- 17 is the privious prime number 17,19,23 so.
because of all this I say that 3 is the last number.
I hope this is ok, if it is, I have to congratulate you for thinking this one that would gave most people a headache for a month(IÂ´m included and this reminds me Lateral Thinking V, the one of the people, the dogs and the boat) After you say yes or no, IÂ´ll think number 1. Would like to see aym thinking this one. Thanks again!