Chicago Cubs First-Place Elimination Scenario
Chicago must win 5 more games (91 total) to avoid elimination from first place.
Here are the final standings in a scenario in which Chicago finishes in first place with 91 wins:

Team             W	L	PCT	GB
Chicago          91	71	0.562	0.00
Milwaukee        91	71	0.562	0.00
Cincinnati       82	80	0.506	9.00
St. Louis        80	82	0.494	11.00
Pittsburgh       71	91	0.438	20.00

Tie breakers
Chicago wins the series with Milwaukee 7 games to 6

To construct the scenario, we start with the current standings in the division:
Team             W	L	GL	PCT	GB
Milwaukee        91	59	12	0.607	0.0
Chicago          86	64	12	0.573	5.0
Cincinnati       75	75	12	0.500	16.0
St. Louis        73	78	11	0.483	18.5
Pittsburgh       65	86	11	0.430	26.5

Next, we show how the remaining series of games plays out in the scenario:

Chicago currently has 5 win(s) against Cincinnati and 4 loss(es).
Chicago wins 2 additional game(s) against Cincinnati and loses 2.
Chicago wins the series 7 games to 6.
Chicago wins 3 additional game(s) against teams outside the division and loses 0.

Chicago currently has 7 win(s) against Milwaukee and 6 loss(es).
Chicago and Milwaukee have no remaining games.
Chicago wins the series 7 games to 6.
Chicago wins 3 additional game(s) against teams outside the division and loses 0.

Chicago currently has 8 win(s) against Pittsburgh and 3 loss(es).
Chicago wins 0 additional game(s) against Pittsburgh and loses 2.
Chicago wins the series 8 games to 5.
Chicago wins 3 additional game(s) against teams outside the division and loses 0.

Chicago currently has 5 win(s) against St. Louis and 5 loss(es).
Chicago wins 0 additional game(s) against St. Louis and loses 3.
Chicago loses the series 5 games to 8.
Chicago wins 3 additional game(s) against teams outside the division and loses 0.

Cincinnati currently has 3 win(s) against Milwaukee and 7 loss(es).
Cincinnati wins 3 additional game(s) against Milwaukee and loses 0.
Cincinnati loses the series 6 games to 7.
Cincinnati wins 0 additional game(s) against teams outside the division and loses 0.

Cincinnati currently has 6 win(s) against Pittsburgh and 4 loss(es).
Cincinnati wins 0 additional game(s) against Pittsburgh and loses 3.
Cincinnati loses the series 6 games to 7.
Cincinnati wins 0 additional game(s) against teams outside the division and loses 0.

Cincinnati currently has 5 win(s) against St. Louis and 6 loss(es).
Cincinnati wins 2 additional game(s) against St. Louis and loses 0.
Cincinnati wins the series 7 games to 6.
Cincinnati wins 0 additional game(s) against teams outside the division and loses 0.

Milwaukee currently has 10 win(s) against Pittsburgh and 3 loss(es).
Milwaukee and Pittsburgh have no remaining games.
Milwaukee wins the series 10 games to 3.
Milwaukee wins 0 additional game(s) against teams outside the division and loses 6.

Milwaukee currently has 6 win(s) against St. Louis and 4 loss(es).
Milwaukee wins 0 additional game(s) against St. Louis and loses 3.
Milwaukee loses the series 6 games to 7.
Milwaukee wins 0 additional game(s) against teams outside the division and loses 6.

Pittsburgh currently has 7 win(s) against St. Louis and 6 loss(es).
Pittsburgh and St. Louis have no remaining games.
Pittsburgh wins the series 7 games to 6.
Pittsburgh wins 1 additional game(s) against teams outside the division and loses 5.

# Chicago Cubs' magic number
# Chicago's magic number
# Cubs' magic number
# MLB Playoff Race