What are some real life applications of the knapsack problems class?
I'll probably try with this one, from an actual software I almost developed.


There's a factory that produces paints in various colours. 
The factory has a single production line that can produce only one colour in a given moment. It's capacity is also limited, i.e. 1000 litres per day regardless of the colour produced.
Different colours have different production costs and are sold at different prices. I.e.: white is cheap to produce and is selling price is low. Gold is expensive to produce because metal sand has to be added in the paint to give it a shiny note. Gold is also sold at higher price. Red lies in between. 
Different customer requires different amount of paint, i.e.: customer A asks for 1000 litres of white, customer B and customer C asks for 250 litres of gold each, customer D for 700 litres of red.


That given, the problem is to find the mix of production that maximise the profit.