One of the best examples so far for me (and it's truly micro) was at grocery store. Friend trying to figure out how big of a rice bag to get and avoid not finishing it before a long trip coming up. She knew she ate a couple of cups dry a week.
"I eat 2 cups dry rice per week. Can I finish a 25lb bag in less than 4 months?" "Yes" (it did show its work).
One shot, perfect response. I know this kind of computational thing is what WolframAlpha was for, but that wasn't nearly as reliable. I know I could figure it out myself, but I'd need to find a reasonable figure for the density of rice and probably do some imperial metric conversions and generally futz around for longer than one would want to stand in front of a pallet of rice bags.