What weight will the fourth scale display? Think carefully! There's a quick and clever shortcut for solving this.
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.