SOLUTION:
Let,
x = cat
y = rabbit
z = dog
x + y = 10
z + y = 20
z + x= 24
z + x+ y = ?
x = 10- y
z = 20-y …… (1)
z = 24-x ……(2)
z = 24-(10-y) ….(3)
z = 24-(10-{-z+20})
z = 24-(10 + z – 20)
z = 24 – 10 – z + 20
2z = 24-10+20
2z = 34
z = 17
z + y = 20
y = 20-z
y = 20-17= 3
z + x = 24
x = 24-z
x = 24-17
x = 7
z + x + y = 17+7+3= 27
Cat= 7kg.
Rabbit= 3kg
Dog= 17kg.
Q.E.D.
‘kovich
madukovich's cogitations 
Very interesting. The only step that nearly gave me goose pimple is: z = 24-(10-{-z+20}) as I was expecting: z to be 20-z. Nether less, I understand the beautiful of Maths from there. QED.
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Hmmmmn, the idea wasn’t to jump the gun, but to carefully outline how every piece came to be what it is, before arriving at “20-z”, for instance.
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