[Davertron]

Does anyone have resources for other shortcut calculations for similar things? This particular one comes up a lot when speaking with my wife's family (they're from Mexico) but I've never tried to come up with a shortcut before. I'd love to see links to other resources, or recommendations for Math tricks for doing quicker mental math.

[I_complete_me]

No resource but there is a similar conversion trick for kilometers to miles. Divide by two and add a tenth. viz: 60 km = 60/2 + 60/10 = 30+6 = 36 miles. Not exact but near enough for many cases.