WRAPPING

It looks to me that @ttokoro  was just cheating and his approach is no real solution to the problem.

I guess he was just copying two separate math regions (salmon pink) in a text region and combined them by a text equal sign (yellow).

You also see that he defined I and V as both being 1 (probably to avoid an error when using the assignment equal := instead of the fat boolean one) which sure would not yield the shown result for Z.
So in my opinion this is not a solution to the problem at all.It just looks like the feature demanded for but actually isn't!

Only benefit is that definition and copied(!) result ride along in one single text region but its no live calculation.

@ttokoro  If my assumptions are wrong, please attach your prime file here. This is generally a better way than just posting pictures, which also often show only part of the game.

The feature asked for simply is not available in Prime and I can't think of a suitable workaround.

BTW, its generally better not to use inline evaluation (evaluation a variable immediately after defining it in the same region)

but rather separate definition and evaluation in two distinct regions:

Additional remark: When you define variable b, you don't have to type 1i*5 but you may simply type 5i (without a space or multiplication in-between!)