Where do you live? Because a complete societal regressiom like the one being talked by OP is not the same that some isolated iletracy or lack of mathematical knowledge is some parts of the world.
Here in my country most people know how to handle their phone and conduct basic mathematics, though there could be isolated areas in the jungle, in the far away states of the republic, where people does not have much access to information or technology or even education, so they do not know how to add two plus two. It is not their fault, it is the system and the government who failed them to provide them with more chances to develop themselves.

What OP is talking about sounds like an exageration to me, while I could agree the western society has gone quite backwards in some aspects, specially in social and economical aspects, to me it is very unlikely for us to reach the stone age, unless a nuclear war breaks out and the fee human beings left would need to start again using rocks and branches as their main tools..