But if you count them, there are indeed 20, there are 10 either side of the carton in the middle..No Rip off there .
The carton is there to separate them as some colours "bleed" very easily, and will contaminate others, especially under pressure of many packs in a box. Unless each colour is wrapped separately in plastic / cellophane.
I used to be an importer here, of many hundreds of kilos of the pro stuff ( much better quality than you purchased ) from the people who made plasticine ( Newclay ) for Ardman studios , the Wallace and Grommit " movies people. they made many other extremely good products too.Unfortunately they did not find a buyer for their business when they retired.They were selling their manufacturing machinery for a while, later their IP ( recipes ) and list of suppliers. Suppliers were all in the UK, I'm not, , profit margin as a manufacturer was not high enough to interest me, and I know how to make almost all of what they made. The products I do not know how to make , did not interest me.
Plasticine, like many other craft products, is sold by weight, not by volume.
The stuff you have will dye your hands and get colour on your clothes, wear vinyl gloves and be careful , the colour can be impossible to get out of some fibres in some clothing.
My son does ( amongst other animation ) , stop motion animation, and we use it for modelling too.
Heating ( double bath, it is inflammable when really warm and pourable ) it will make it softer, you can heat to to be pourable . If it goes hard you can "bring it back" by adding a few drops of vaseline oil ( or lily white oil or light cycle oil or sewing machine oil ) and mixing it in by hand. to keep it soft and "workable" put it in a foil dish and rig an incandescent light bulb of about 40w just above it. The heat from the bulb will keep it soft-ish
