exactly, the most easy way to solve this is to add up the first 2 pictures together. which equals to picture 3 + 2 rabbits. aka 24 + 2 rabbits. meaning that 2 rabbits are 6 kg together. so picture 3 + 6 kg = 27 kg
this way you dont need to even bother calculating how much a dog or cat weighs. even though it wouldnt be that hard to calculate now.
In the first 3 pictures, each animal appears twice. If you add their weight together and then take half of that total, you get how much all 3 weigh together, without actually calculating how much each of them weigh individually.
step 1: x + y - 10 = 0 = x + z - 20
step 2: solve above --> y - z + 10 = 0 = y + z - 24
step 3: 2z = 34
dog = 17 = z
step 4: Substitute z:
z + x = 20
17 + x = 20
Bunny = x = 3
Step 5 substitute x:
x + y = 10
3 + y = 10
cat = 7 = y
I feel like it is simpler just to say:
2(b+c+d)=54
2 because each animal is weighed twice in the picture, and 54 is the total of the three weighings
b+c+d = 54/2
=27
then you know that the bunny must be 3 kg because the weight of the dog/cat combo was 24... (27-24=weight of bunny), cat=10-3 ... so 7, and dog=20-3 so 17
Both ways get the same answer though, just this seems clearer in my head