Answer:
Nina could buy 36 old games if she buys 1 new game.
Nina could buy 24 old games if she buys 4 new game.
Explanation:
Cost of older games = 500 points
Cost of new games = 2000 points
Nina uses the equation:

to download games, where a is the number of older games, b is the number of new games.
a) Nina downloads 1 new game.
Thus, b = 1
Putting this value in the equation:

Thus, Nina could buy 36 old games if she buys 1 new game.
b) Nina downloads 4 new game.
Thus, b = 4
Putting this value in the equation:

Thus, Nina could buy 24 old games if she buys 4 new game.