Find (4.54 x 10^8) - (3.98 x 10^8). When you regroup the decimals,
what do you notice about their difference? How does this affect the
exponent of the difference?

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.