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1.
For this problem, 3/5 of cheese cake had same mass as 3/4 of marble cake.
That is convenient because the units matched (both 3). So we can draw this:

Cheese cake : U U U U U    (3/5 in bold)
Marble cake : U U U U      (3/4 in bold)

So 5U = 2 kg. Single U = 2/5 kg. Marble cake = 4U = 8/5 kg = 1 3/5 kg.
(Not sure if they consider 1 3/5 a mixed fraction?)

2.
This one is different from the above. We cannot just use "3 units" because 3
appeared in both question. Here the "3" is David's money and it changed bcos
he spent it. What did not change is Samy's. So, really the unit we have to
focus on is Samy's.

          David     Samy
Start       3    :   4
            6    :   8    (because 3:4 = 6:8)
End         3    :   8

So the model:
   Samy          : u u u u u u u u
   David (start) : u u u u u u
   David (end)   : u u u

We see that 3u was spent (the one highlighted). So u = 144/3 = 48.
At first they had 6u + 8u = 14u. 
So they had 14 x 48 = $672 altogether at first.

CHECK:
   Samy:           8 x 48 = 384
   David (start) = 6 x 48 = 288    (288:384 = 3:4)
   David (end)  = 288-144 = 144    (144:384 = 3:8)
   
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