In profit &loss; test 15 in question number 4 if discount of 25% is due to 9 jeans then 100% discount equals to 36 jeans then 15 shirts become 36 jeans.is this right approach?

No ,  this is not the right approach . Discount is calculated on Total Bill. When you buy shirts, you got 9 jeans free. So the discount value is the total cost of 15 shirts and 9 jeans and that is 25%. Basically 100% discount means you are taking everything  free of cost, which is not the case here...