Originally Posted by ColinsMum
Yes, you're wrong, because you are assuming that such a number must be an approximation. It need not be; it might be a precise representation of a rational number, in which case appending a zero gives you a different representation of the same rational number, and is definitely correct.

This.

This is pure mathematics, and these are just numbers. 3.1416 and 3.14160 are exactly the same number.

Looking at the sum 3.1416+2.71828 and seeing some issue with measurement/precision/significant digits, is a bit like looking at a slice of burnt toast and seeing an image of the Virgin Mary. Its pedagogically unsound.