A trader sells an article at 20% profit. If he buys it at half of the cost price and sells at the same selling price as before, then what will be the profit percentage?

140%

If the cost price is 100 units,

At 20% profit, selling price = 120 units 

Now, if cost price is halved,

New cost price = 50 units and selling price remains 120 units,

Profit = 120 - 50 = 70 units 

Profit % = $\frac{70}{50}$ x 100 = 140%