Introduction:
In this scenario, a merchant has changed the trade discount from 20% to 25%. We need to determine the percentage change in the selling price so that the amount available to the trader remains unchanged. To solve this problem, we will first calculate the original selling price and then find the new selling price.

Step 1: Calculate the original selling price:
Let's assume the cost price of the item is $100. With a trade discount of 20%, the selling price would be calculated as follows:
Selling price = Cost price - (Trade discount * Cost price)
Selling price = $100 - (0.20 * $100)
Selling price = $100 - $20
Selling price = $80

Step 2: Calculate the new trade discount:
The merchant has changed the trade discount to 25%. We can calculate the new selling price using the following formula:
Selling price = Cost price - (Trade discount * Cost price)
Selling price = $100 - (0.25 * $100)
Selling price = $100 - $25
Selling price = $75

Step 3: Calculate the percentage change in the selling price:
To find the percentage change in the selling price, we can use the following formula:
Percentage change = (New value - Old value) / Old value * 100
Percentage change = ($75 - $80) / $80 * 100
Percentage change = -$5 / $80 * 100
Percentage change = -0.0625 * 100
Percentage change = -6.25%

Conclusion:
In order to keep the amount available to the trader unchanged, the selling price should be decreased by 6.25%. This means that the merchant needs to reduce the selling price by 6.25% to offset the increase in the trade discount from 20% to 25%.