Each pack of dvds costs w dollars. If there is a discount of x dollars on each pack How much do you have to pay to buy each pack?
Let's say we have one pack of DVDs. The cost of this pack is w dollars. With the discount of x dollars, the cost of the pack becomes w-x dollars. 

Now let's say we have two packs of DVDs. The cost of the first pack is w dollars, the cost of the second pack is w-x dollars. The total cost of the two packs is 2w-x dollars. 

Now let's say we have three packs of DVDs. The cost of the first pack is w dollars, the cost of the second pack is w-x dollars, the cost of the third pack is w-x dollars. The total cost of the three packs is 3w-3x dollars. 

Now let's say we have n packs of DVDs. The cost of the first pack is w dollars, the cost of the second pack is w-x dollars, the cost of the third pack is w-x dollars, ..., the cost of the nth pack is w-x dollars. The total cost of the n packs is nw-nx dollars. 

We can see that the cost of one pack is w-x dollars.