I think this is correct:
rabbit = 20g - dog
cat = 10g - rabbit
dog = 24g - cat
rabbit = 20g - (24g - cat)
rabbit = 20g - 24g + cat
rabbit = 20g - 24g + 10g - rabbit
2 rabbits = 6g
rabbit = 3g
dog + cat = 24kg
dog + cat + rabbit = 27g
cat = 7g
dog = 17g
(10 +20+24)/2. 27
Called simultaneous equations, IIRC