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?
Do want to have an average profit of $4 for each shirt or do you want to ensure that all shirts are sold for at least a profit of $4?
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.