"The objective in ArithmeTick is to solve as many addition, subtraction, multiplication, and division problems as possible before time runs out."
From the makers of SpaceTime, one of my favorite applications on my Pocket PC, is a fun little game to see how quick (or slow) your brain is. It runs on WM5 as well as older devices. I tried it out on my BlackJack and it works quite well. Best of all, it is free!