What weight will the fourth scale display?

tl;dr: The first three scales combined have 2 of each animal, so sum up those scales and divide by 2.

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In the first scale, $R+C = 14.$ In the second, $R+D = 26.$ In the third, $C+D = 30.$

Now, we could solve these equations...but all we need is $R+C+D.$ Adding these equations gives $(R+C) + (R+D) + (C+D) = 2R + 2C + 2D = 70,$ so $R + C + D = \frac{70}{2} = 35.$

Putting all the animals on the same plate and we obtain a total weight of 70 (two rabbits, two dogs and two cats) Thus the result is the half of 70 that is 35.

I saw the easy method, but chose to do a more elaborate one. The first scale say bunny(b) + cat(c) = 14, so b= 14-c. The second scale says dog(d) +b=26. Substitute and d + 14-c = 26. Simplify to d-c = 12. Add that to the third scale equation d +c =30. You get 2d = 42 and d=21. Then d+c=30 gives you c=9, and d+b=26 gives you b=5. So d+b+c= 35. Bonus: you know exactly how much EACH animal weighs! :-)

Scale 1 : rabbit and cat Scale 2 : rabbit and dog Scale 3 : cat and dog Add scale 1,2,3 and we get 2 rabbits + 2 cats + 2 dogs = 70 then divide by 2. We got 35.

Since the cat with the dog weighs four more than the rabbit and dog, c=r+4. Therefore 2r+4=14, and the rabbit weighs 5. Since we know the other two animal weights combined is 30, we add 30+5 to get a total weight of 35 or choice B.

I added the 2nd and 3rd scales for 56 (Bunny, Cat, and 2 dogs). Now subtract the 1st scale for a new total of 42 (two dogs). Divide by 2 to arrive at the scale for 1 dog, 21. Now add back in scale 1 to arrive at 1 dog, 1 bunny, 1 cat for a final scale of 35.

(Bunny + Dog=26) + (Cat + Dog=30) = 2 Dogs + (Bunny + Cat=14) or 56 = 2 Dogs + 14. Subtract (Bunny + Cat=14) and 56-14=42 for 2 Dogs, then each Dog is 21. ?=(Dog=21) + (Bunny + Cat=14) for a total of 35.

I used the first scale R + C =14 and noticed that the blue scale was D + 14. Next I noticed the middle two scales were 2D + R + C =56. Substituting for R + C yields 2D + 14 = 56 therefore D = 21, and 21 + 14 = 35.

TO FIND OUT THE WEIGHT OF THE DOG:

*Dog and Rabbit = 26

*Dog and Cat = 30

Two Dogs, Cat, and Rabbit = 26 + 30 = 56

*Cat and Rabbit = 14

Two Dogs - Cat and Rabbit = 56 - 14 = 42

*One Dog = 42 ÷ 2 = 21 <-- Yay!

COMBINED WEIGHT:

Dog, Cat and Rabbit = 21 + 14 = 35

Shortcut: Add all three to get 2 x (rabbit, cat, dog) to get 70. Divide by 2 to get 1 x (rabbit, cat, dog) 35.

No need to solve for the individual animals to answer the question (but I did that anyway before I noticed the trick, but at least I can still manage do that in my head, small comfort).