Mental Math

No equation, no formula, no calculator, no writing on paper, just do mental calculation.

What is the answer  in the last diagram ?

Scroll down to see the answer, but not until you have tried yourself.
(There are many methods, but one is the fastest and the best !)
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Ans:
Add the first 3 diagrams: 10+20+24 = 54 for 2x (Dog + Cat + Rabbit).

Dog + Cat + Rabbit = 54/2 = 27