This question blow up my mind pls someone explain this

Question

!This concept is taken from mathematical circle Pls try to explain as easy you can

Answer 1

There are seven numbers from $0$ to $6$. And every number appears twice as on a double domino, and once on a domino matching it with every one of the six other numbers. So each number appears eight times. (And hence there are $\frac {7*8}2 = 28$ dominos.)

In chain each number is matched with itself in pairs. Ever number appears and even number of times within the inside of the chain. If the ends of the chains were different that those two numbers would appear an odd number of times but that's impossible as each number must appear $8$ times. So the ends of a chain must be the same number. So if one end is $5$ so is the other.

Now if you remove the $0$s then there are $6$ numbers but the each appear $7$ times. $7$ is an odd number so if you attempt to make a chain all the numbers on the inside must pair up. But for all the numbers not at the end there are an odd number to pair up so that is impossible. You can not make a chain with those dominos.