Let assume the cost price is 100%Marked up price + cost price = 20 + 100 = 120%Discount at 20% = 20/100 × 120 % of cost priceSelling price = cost price − gain= (120 − 24)% of cost price= 96% of cost priceLoss = (100− 96)% of cost price= 4% of cost price∴ He will wake 4% loss