One decametre = 10 m
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651 m = 65,100 cm is OK because there are 100 cm in a metre.
However, to continue on, go back to the metre, then (at least in your mind) do it one thousand at a time
metre, millimetre, micrometre, nanometre
three steps, each with three zeros
Going from milliltres to litres, then back to microlitre is a trap: it goes one way, then the other.
Do it in the proper sequence: litres, milli, micro (take the number of millilitres, slap three zeros)
km to mm to nm is skipping steps, so mentally fill them in
km - m - mm - μm - nm
The last two are also traps (changing direction). Just rearrange them in the right order (at least, in your mind)