Answer:
upper
Step-by-step explanation:
Zone of Proximal Development or ZPD is a concept developed by Vygotsky. It means the distance between what a person can do alone and what he can do with the help of or guidance from a skilled person.
ZPD has two limits. A lower limit, which is the things that are difficult but possible for a person to learn alone, and an upper limit i.e. the things that a person cannot learn, even with assistance from an adult.
Here, Johnny cannot learn algebra even with an expert, so learning algebra is in the upper limit.