A good approximation is ... divide the number by 7, then ...
from kts to mph add what you get to get mph, or
from mph to kts, subtract what you get to get kts.

ex .. 70 kts is 70 plus 10 or 80 mph.

or 70 mph is 70 minus 10 or 60 kts.

It's close, within a knot or two ( or mph or 2)
not exact ..but ..close nuf for government work !
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Cheers, Charlie