Final answer:
To calculate the percentage gain, subtract the initial cost from the final selling price, then divide the gain by the initial cost and multiply by 100.
Step-by-step explanation:
To calculate the percentage gain, first we need to find the gain in value by subtracting the initial cost from the final selling price: 
Gain = Final Selling Price - Initial Cost 
Gain = $22,500 - $18,000 = $4,500 
Next, we calculate the percentage gain by dividing the gain by the initial cost and multiplying by 100:
Percentage Gain = (Gain/Initial Cost) * 100 
Percentage Gain = ($4,500/$18,000) * 100 = 25%