I participated in an Estimathon (a speed contest of Fermi problems) not long ago. It works as follows. Contestants are given questions and they must give a closed range $[a,b]$ which should contain the correct answer. The scoring guidelines are such that you want to minimize your "number of points" where the points depends on $\left\lfloor\frac{b}{a}\right\rfloor$. If your range does not contain the correct answer, your number of points doubles which is bad. So the top priority is to make sure your range contains the correct answer. After that, it is important to make sure your range is not too big.
One of the questions was:
Estimate $100!$.
I was the only person among 40 people to get this correct, since I had memorized $100! \approx 9.33 \times 10^{157} $, so I just put $\left[9 \times 10^{157}, 10^{158}\right]$. However, is there a way to get this correct to one order of magnitude? Note that two orders of magnitude is too imprecise for this contest.
I was thinking of using Stirling's approximation but even that is quite tedious to do by hand (no calculators were allowed)!