3. arnie's car used 100 cups of gasoline during
a drive. he paid $3.12 per gallon for gas.
how much did the gas cost?

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.