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 fa

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).