Seeing that four cats and three kittens weigh 37 pounds while three cats and four kittens weigh 33 pounds, we are raised what is the weight of cats and kittens.
We can verify that the upper scale contains one more cat and one less kitten than the lower scale and the difference is 4 pounds.
One of the lower scale kittens suddenly becomes a cat and gains 4 pounds of weight, therefore the difference in weight between a cat and a kitten is 4 pounds.
If we change all the cats of the upper scale by kittens then we would have 7 kittens and 16 pounds balance with 37 pounds.
Now we remove 16 pounds from both ends of the scale and we have 7 kittens balance with 21 pounds which shows that each kitten weighs 3 pounds and therefore each cat weighs 7 pounds.