What weight will the fourth scale display?

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What weight will the fourth scale display?

Think carefully! There’s a quick and clever shortcut for solving this.

Correct!
Wrong!

Let
R, C, and D be the weights of the rabbit, cat, and dog, respectively. Each picture lends itself to an equation, giving us the following system:

R + C​ = 10,
D + R = 20,
C+ D = 24,
C + D + R = ?
If we add these equations together, we get
2C + 2R + 2D = 54.

If we divide by 2, we are left with the sum of the weights of each animal:
C + R + D = 27.