Our innate mental ability to deal with quantities is very poor. We are probably not much better computers than many other animals. However, through procedural thinking, we have learned to do much more than, say, apes.
Our troubles with quantities starts with the fact that we can't perceive and remember them. Computing devices like the one I describe here are enormously helpful because they hold quantities. Hence, they allow to use the result of the previous operation in the current one. In modern computing, such devices are called accumulators.
Numerous examples of accumulating include voting, producing, fund raising, wealth building, weight losing and running virtually any business, from a household to a bank. Teach your student these scenarios. Start from performing singular long additions and move to adding several long numbers at once. Normally, kids appreciate us being serious with them.