If we take 90 points needed for the title then City would need to get 67 points from 25 games. That's 2.68 points per game.
Thats the equivalent of a 102 point season. So for city to get to 90 points they need to go on a run of form that's better then they've ever been.
To put it another way. For 90 points, City can only drop 8 points in next 25 games.
For Arsenal to hit 90 points they need the equivalent form of a 98point season. They can only afford to drop 10 points in 25 games.
For comparison, we could drop 19 points and still hit 90.