<!-- a thief (or burglar) steals, a robber robs (confronts a person while stealing). Not exactly a perfect definition, but close enough. -- Gerard Schildberger. -->
The thief knows the weights and prices of each items. Because he has a knapsack with 15 kg maximal capacity, he wants to select the items such that he would have his profit maximized. He may cut the items; the item has a reduced price after cutting that is proportional to the original price by the ratio of masses. That means: half of an item has half the price of the original.