Can explain why are there two 6/15 in the first model and what is u? Thank you.

Just put it this way.
Ben have 15p
(common multiple of 3 and 5)
Marcus have 15p + u ( we don't know how much more Marcus have than Ben, so is u, unless they received same amount, then no need u))
...

If Ben gave 2/5 to Marcus, meaning gave 6u to Marcus
Marcus have 12p + u more than Ben
(Agreed?, Draw model if you cannot imagine).
12p + u = 64

If Ben gave 2/3 to Marcus, meaning gave 10p to Marcus.
Marcus will have 20p + u.... Can see?
20p + u = 144

The difference
20p + u - 12p - u = 144 - 64
8p = 80
p = 10

Ben at 1st = 15p = 15×10 = 150

That all for explanation. : ) Hope you can understand.

But still don't understand why when ben gives 6p to Marcus, Marcus will have 12p + u instead of 6p + u?

Oh...
You have 4
I have 4

You give 1 to me,you left 3
I have 4 +1 = 5, I have 1+1 more than you....

We find the difference ...
Find how much Marcus has MORE THAN Ben.

I am crystal clear now. Thanks a million.