Calculate a baseball player's batting average. Calculating a baseball player's batting average is not a difficult task at all. A batting average represents the percentage of at bats that result in hits for a particular baseball player. The formula is: That's all there is to it.
To fly under the radar means to act in a way so as to actively avoid detection. It is the opposite of 'in your face'. There is an element of getting away with something but it is not necessarily bad.
An airplane's radar is a bit like a torch that uses radio waves instead of light. The plane transmits an intermittent radar beam (so it sends a signal only part of the time) and, for the rest of the time, listens out for any reflections of that beam from nearby objects.