Problem with solving equation !!!

Hi all!

I was working on project and found a big problem.

Example:

I have function: 10+x=15 and i need to find x.

Its how i do in Mathcad:

10+x=15 solve,x -> 5

now i want to use x in further calculations.

x+2=

Mathcad doesn't calculate it, it makes x in red color and says that it is undefined.

Same thing happens if i do it with:

Given

10+x=15

Find(x)->5

So bacically what would i do, is:

10+x=15 solve,x -> 5

And now i define x.

x:=5

and write my expression:

x+2=7

now it calculates it.

But when i do research work and i have 20 pages of calculations and many similar cases, where i manually define x or another symbols, and then i change something in the begining, so i have to go to all that manually defined values and redefine them, it is unacceptable!

Is there any solution or function which would make solutions of equations and use calculated values in further formulas?

I attached mathcad file, for you to see what happens. If you know how to do it, i would like you to make mathcad example to, or just explain.

Thank you!