If the Yankees play 162 games, 19 against the Sox and 143 against everyone else:
They could possibly go 143 - 0 against every other team and it not affect the Sox.
The Sox could go 143 - 0 against every other team and it not affect the Yankees.
There are 19 games against each other. If both teams went 143 - 0 otherwise, they would win the division by going 153 - 9.
Now, we are 4 games into the season. The O's have the best record at 3 - 1. The Sox are at 1 - 3.
If the Sox and O's win every game against other opponents going forward, the O's will be 145 - 1 and the Sox will be 143 - 3 (16 games left to play against each other). Sox need to win 10 games from the O's, so they would have to get to 153 - 9 to win the division. So the magic number is 152?