$ \LaTeX $ for fun and...well, fun.

To put in math, put it between $ signs, or use \[...\], \(...\), or $$...$$.

Say you've had an infinite number of cups of tea (close enough), each six hours after the previous one and the last one just now. Each has about 50 mg of caffeine. The half-life of caffeine is about six hours, so the number of milligrams of caffeine in your bloodstream is now:

$$ 50 \textrm{mg} \sum_{n=0}^{\infty}
\frac{1}{2^n} = 100 \textrm{mg} $$

Of course, in reality caffeine enters the bloodstream gradually; adjusting for that is left as an exercise for the reader.