To get your answer to the math equation, here are the steps --
55 posters for sale
Out of the 55 posters for sale, 34 of them show sports.
You are now left with the remaining posters from the 55 -
If the remaining posters show animals, here is your math problem.
55 - 34 = 21
21 posters show animals
Pretty easy, right? It's only subtraction!
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Hope this helps! :D