Do the Math
"Do the Math" basically means to use critical thinking or reasoning rather than believing someone else's assesment.
The Evidence
"The math" is the evidence you have, and the "doing" is examination of it to draw your own conclusion, but this isn't harsh. I use it because it might otherwise be easy to debate or negate my own conclusions.
Notice the Obvious
Math is math, and there is no "interpretation" of it. It just is. "Do the math" then is another way of saying "notice the obvious."
If a child says "I didn't do it," when you ask about the missing chocolate, then "do the math" would mean to look at the evidence (chocolate on their hands and around their mouth) rather than the child's pronouncements.
Same here.
Your Own Truth
The truth that matters the most is our own, and therefore I prefer that you do your own research (class exercises) to discover what is true for you, rather than taking my word for what I believe is true. Besides, you might see, feel or notice more than I do.
Include the Body
"Doing the math" also means to include all gathered information, including your own body sensations, when you make a deduction, have a realization or make a choice. How our body feels (an emotion, a blood pressure change, a muscle contraction, etc.) is information that should always be included when we make a choice. Your body never lies.