The PCs come to an inscription in a wall that reads:
Give thou the ages of four from youth, and then an elder's age, forsooth.
Beneath this, there are four dials with numbers ranging from 1-20 and one dial with numbers ranging from 1-100.
Once the PCs turn the dials so that each is set to a number, the following inscription appears, and under that, a surface for writing on (in some manner).
Each of the younger must once combine to equal the elder's span of time.
[sblock=solution] This is an obfuscation of a game a friend introduced me to years ago, wherein you take four numbers between 1 and 20 and one number between one and 50 (you can use higher numbers for more difficulty) and use each of the first four numbers once and only once in an equation to equal the last.
Depending on the numbers, the solution(s) might be simple, or frustratingly hard. With a very wide range of numbers, there might not be any solution at all (so, probably stick within the lower ranges for your game).
This particular puzzle would be good to use at an intersection, or a place that the PCs will return to often, because, as long as it requires a different set of numbers each time, the PCs will have to come up with a new solution each time they want to bypass it.
A warning though, I would not use this puzzle unless I knew that my players would enjoy the math-based challenge! Otherwise, you'll be facing some serious frustration![/sblock]