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:
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.