Mental math can give you a quick idea of whether or not something is worth investigating more closely. I play a guessing game at the grocery store after I load

Mental math can give you a quick idea of whether or not something is worth investigating more closely. I play a guessing game at the grocery store after I load my cart onto the belt and am waiting for the checker to finish. I’m usually within 5% when there’s a large variety of things and produce is involved, and have been alerted that I needed to check the receipt more carefully than usual. This habit helps me be more confident at restaurants in a place where I’m a second language speaker - and when discussing pricing with vendors in that second language.