Sunny 16 works fine, but it's less useful at lower light levels when the eye compensates, making it difficult to judge exposure. Last summer I was shooting pre-sunset and I compensated half a stop, when in fact the light level was dropping exponentially. So basically, if you have a modern high latitude film, and an eye for haze, smog and the difference between shadow and full sun, the s16 rule works well.
Thank you everyone! But really my original question doesn't really deal with how to determine exposure.
The original question is somewhat unclear. Is the student asking how shutter speeds and apertures relate to one another (logarithmically)? Or do they need to understand the inverse square law (double the distance, square the exposure)?