Explain how knowing 1 + 7 you find the sum of 7 + 1

They are  backwards so you can use them like that but just not with subtraction 

plz give thanks : )

It helps you because it is the same the only that is different is that the 7 is first

The first one, i believe, would be correct. all the others have one thing in common, and that's that they have repeating numbers. in the second one, x and y have repeating 6's and 3's, and in the third, x and y have repeating 6's as well. in the last one, there are repeating 3's. the first one has no repeating numbers.