How does more employees change wait time in the New Licence Challenge

Question

https://code.sololearn.com/cz5YWILkuM06/?ref=app

Answer 1

------------- 0 Minutes
A B C D
X X
------------- serving A + B
C D
X X
------------- 20 Minutes (A + B finished)
C D
X X
------------- serving C + D

X X
------------- 40 Minutes (C + D finished)

Answer 2

Alex I'm sorry but i still do not understand the grouping thing. It might be my basic understanding of math or something that's foundational. Even when I tried doing groups by Adding Math.Floor() which makes sense more than Math.Ceiling() it still didn't work but Math.Ceiling worked done how