Zequals and the Art of Estimation
This Numberphile video is about Zequals which can be useful if you need to compute a quick estimate, for example to find out if your calculation is on the right track.
The idea is to ruthlessly round every number to just one significant digit. For example:
3 * 7 = 21 zz 20 7 * 8 = 56 zz 60 436 * 68 zz 400 * 70 = 28000 zz 30000
The actual answer is 29648 and surprisingly close to the zequal-result. Not all numbers turn out to be that close, but this method works really well for some ranges as this graph illustrates. There’s also extra footage and more to read about zequals on Brady Haran’s Blog.