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).
I had an Asus laptop that would only pickup wifi if I hibernated it first. I have a Dell touchscreen model that took a full day to get the touchpad to work at all as Ubuntu kept defaulting to the screen. Too afraid to do a fresh install of the current (or any other) distro because I can't remember how I fixed the touchpad issue.
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).