How do you convert $15$ nm into km?

Hint:In order to convert nm into km, use the following relations between nm, m and km
$
1nm = 0.000000001m \\
1m = 0.001km \\
\Rightarrow 1nm = 0.000000001 \times 0.001km \\
\therefore 1nm = 0.000000000001 km \\
$
Where nm, m and km are known as nanometer, meter and kilometer respectively.
Use the relation $1nm = 0.000000000001 km$ to convert $15$ nm into km.

Complete step by step solution:
In order to convert $15$ nm into km, we should know about the relation between these km and nm.
Since we know that $1nm = 0.000000001 m\;{\text{and}}\,1m = 0.001km$
From above information we will derive the relation between nm and km
$
\because 1nm = 0.000000001m\;{\text{and}}\,1m = 0.001km \\
\Rightarrow 1nm = 0.000000001 \times 0.001km \\
\Rightarrow 1nm = 0.000000000001km \\
$
We have to multiply it by $15$ both the sides in order to convert $15$ nm into km
Multiplying by $15$ both sides, we will get
$
\Rightarrow 15 \times 1nm = 15 \times 0.000000000001km \\
\Rightarrow 15nm = 0.000000000015km \\
$
$\therefore 0.000000000015km$ is the converted form of $15nm$

Note: The Standard International (S.I.) unit for measurement of length or one dimensional (1-D) physical quantity is “Meter”, which is represented by the symbol “m”, “nanometer” and kilometer are parts of it that is they are units of length in metric system, where the symbolic representation of nanometer and kilometers are “nm” and “km” respectively.

One nanometer equals one billionth part of a meter and a kilometer consists of a thousand meters. That is $1nm = 0.000000001m\;{\text{and}}\,1m = 0.001km$ in mathematical representation.

Nanometer scale is used at micro levels for measuring length of proton, electron, bacteria, etc. whereas kilometer scale is used at mezzo or normal level for measuring distance between cities, etc. There are some more units around the world, but meter scale is the only standard internal system for measurement of length. One meter is defined as follows

“The length of the path travelled by light in vacuum in $\dfrac{1}{{299792458}}$ of a second”