Sentence:  “The child has marks all over. I saw them when I took her to the bathroom. Ian, she is hurting this child. Now I don't know for sure what's going on here, and maybe it's none of my business. Nevertheless, even I can <WSD>do the math</WSD>. ... You can't let her keep this child for one more day.” 
Meaning of the word marked with <WSD> tags:  To make a thoughtful effort to determine the motive for someone's behavior; to develop one's own understanding of the explanation for or precise nature of a situation.