Not getting too fancy about it, but you start with a base rent of $550 and have 96 apartments. So the total income is which is $1800 short of expectations.

So by increasing the rent by $25 we lose 3 apartments. So the income may be calculated by:

where n is the number of $25 steps the rent has increased by. Numerically we have:

Since we need an income of $54600 we will need:

or

Thus n = 4 or 6.

So we are using either 4 steps of $25 or 6 steps of $25. Which value should you choose? I'll leave that one open to the more business minded people than me.

-Dan