If I understand you correctly, you are wondering why it doesn't print .2 in the first case? It's because a*10 doesn't change a, it returns a new DecimalType. If you instead run echo a*10 you should get what you expect. If you want to save it you have to assign it to a variable like this: var b = a*10.
a *= 10 on the other hand DOES CHANGE a. That's what "inplace" means. So the difference between * and *= is that the first one does not change a, but the other do. Have I understood you correctly? :-)
