I work in big data/cloud stuff so I do estimates all the time to figure out what size resources I need/how long something will take. Less than 5 minutes ago I did mental math to see how long a job would take (I knew how long it would take for one read, and I had 2 days of reads over 3 data sources each with 8 data partitions => 48 reads each taking about 90 seconds => about 1.25 hours).

Tip is another thing. And I like to "min/max" grocery shopping so I typically do mental math when shopping to maximize things like grams of protein/$.