>1.6 is the factor everyone talks about (approximation of 1.609), but it has 2 significant figures. To make mental calculations easier/quicker I use its reciprocal, 0.621, which I approximate with 0.6, which has only one significant figure, 6.How about just 1.5 + 0.1? I.e. the same amount plus half, plus a tenth?So, 55 mi -> 55 + (25 + 2.5) + 5.5 -> 88kmAnd inversely, 0.5 + 0.1, so half plus a tenth:80km -> 40 + 8 -> 48 milesAnd for most purposes, just (one and a half = 1.5) and (half = 0.5) is close enough. You can mentally add a little more.So55mi -> 55 + (25 + 2.5) -> 82 (let's say around 85)80km -> 40miles (let's say around 45 miles)

 I often do mental math like this. However, sometimes it is faster to not split up calculations like this. For instance, in the case of converting 55 mi, it's faster (at least, for me) to just divide 54 by 6, move the decimal, and call it a day.I've found that calculations are faster when you know when and when not to split calculations like the way you describe (even though at least 80% of the time it's faster to split them up).
 This is exactly the way I do math in my head.

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