if you sell the shirts for P initially, then you will sell them for .6P during the sale.
You purchased 100 dress shirts for 20 bucks each, after 1 month of selling the shirts at regular price, you plan to have a sale of 40 percent off original selling price. However you still want to make a profit of 4 dollars on each shirt at sale price. What should you price the shirts initially to ensure this?
If the former, there is not enough information and you should post the whole question. If the latter, then the answer is obviously $24.
If on the other hand you want to make a profit of exactly $4 on every shirt sold at sale price, then there's not enough information - the regular price of the shirt needs to be known.