0.001 gm of carbon is required to write a letter with a graphite pencil.  The total number of carbon atoms used in writing the letter is

12g of carbon-12 represents one mole of the substance. So it contains $6.023\times {10}^{23}$ carbon atoms.

$12\text{\hspace{0.17em}g\hspace{0.17em}of\hspace{0.17em}C-}12\text{\hspace{0.17em}}=6\times {10}^{23}\text{\hspace{0.17em}of\hspace{0.17em}C-12\hspace{0.17em}\hspace{0.17em}atoms}$

$0.001\text{\hspace{0.17em}g\hspace{0.17em}of\hspace{0.17em}C-}12\to x$

Then number of C-12 atoms present in 0.001 g of C-12 is

$x=\frac{0.001\times 6\times {10}^{23}}{12}=5\times {10}^{19}\text{\hspace{0.17em}of\hspace{0.17em}C-12\hspace{0.17em}\hspace{0.17em}atoms}$