> Also the research was more along the lines of "the sodium contained in msg does not contribute to raised blood/system sodium levels"

That doesn't sound very plausible. When MSG dissolves, it dissociates into sodium ions and glutamate ions. The sodium ions are exactly the same as the ions from, e.g., sodium chloride - they don't "remember" that they used to be attached to glutamate ions. And surely any MSG that you eat will dissolve pretty quickly as it goes through your digestive system?

The controversial health claims about MSG are that it has some other mysterious effect that makes people feel ill after eating it, in a way that doesn't happen with sodium chloride. Sometimes it is claimed that everyone experiences these effects, but sometimes it is claimed that only certain people are sensitive to it (which, of course, would be harder to rule out).

> Real numbers are those which can be represented on a number line. As per definition

That isn't really the definition of real numbers - it's just a way of thinking about them. Real numbers are usually defined in terms of sets or sequences of rational numbers (fractions) - you can google "Dedekind cut" or "Cauchy sequence" if you want the full details, but they're maybe a bit beyond ELI5.

> but if we don't know their exact precise value then how can we plot it?

On a real-life number line, we don't really know the exact position of any number, since we don't have any perfect measuring devices. We can use real numbers to model reality in approximate terms, but they don't correspond exactly to reality. For example, we know that there are infinitely many real numbers between 0 and 1, but are there are infinitely many points between two distinct positions in space? Nobody really knows.