If you reduce or remove a non value adding activity the overall outcome won't change, but the process time will be reduced. Often the non value adding activities take some time, but they can be caused by unnecessary transportation, doing things again etc. In Lean production they are called waste. A couple of examples:
- corrections: when you have to fix things
- over-production: when you do more than you need
- over-processing: when something can be done in a simpler way
- transportation: when you move stuff around unnecessarily
- motion: when you have to move around unnecessarily
- inventory: when you have to store something
- waiting: when you have to wait for somebody or something
These are all non value adding activities.
For example, if you have a doctors appointment, when you wait for it, it is a non value adding activity from your point of view. Another example, if you have a small kitchen and you have to move stuff around all the time to be able to make breakfast, that is also a non value adding activity, however it has nothing to do with time; it is a transportation waste.
I think your example is some kind of an exercise and I don't want to solve it instead of you, but when something sits or waits is a waste.