Merger Technical question?

The guides say that when analyzing a company acquiring another, to calculate EV and EQ just add the market caps and adjust for cash, debt, and stock used. Yet, other people say adding market caps is wrong and you need to do something else. Also, can someone solve the below question assuming A buys B in an all stock transaction. I would really appreciate if someone can tell me how these types of problem are supposed to be done.  Here is the example from the guides: 

• Company A: Enterprise Value of 100, Market Cap of 80, EBITDA of 10, Net
Income of 4.
• Company B: Enterprise Value of 40, Market Cap of 40, EBITDA of 8, Net
Income of 2.

Company A decides to acquire Company B using 100% cash.
What are the combined EBITDA and P / E multiples?
In this scenario, you add the Market Caps of both companies together and then
adjust for the Cash, Debt, and Stock used.
Combined Market Cap = 120. Previously, A had 20 more Debt than Cash, and B
had the same amount of Cash and Debt.
To get real numbers here, let’s just say that A had 60 of Debt and 40 of Cash.
Afterward, the Debt remains at 60 but all the cash is gone because it used the
Cash to acquire B. We don’t need to look at B’s numbers at all because its Cash
and Debt cancel each other out.
So the Combined Enterprise Value = 120 + 60 = 180.
You add the EBITDA and Net Income from both companies to get the combined
figures. This is not 100% accurate because Interest Income changes for Company
A since it’s using cash, but it’s small enough to ignore here:
• Combined EV / EBITDA = 180 / (10 + 8) = 180 / 18 = 10x.
• Combined P / E = 120 / (4 + 2) = 120 / 6 = 20x.