c
$4^{\text {th }}$ division coincides with $3^{\text {rd }}$ division then
$ 0.004 \mathrm{~cm}=4 \mathrm{VSD}-3 \mathrm{MSD}$
$49 \mathrm{MSD}=50 \mathrm{VSD}$
$1 \mathrm{MSD}=\frac{1}{\mathrm{~N}} \mathrm{~cm}$
$0.004=4\left\{\frac{49}{50} \mathrm{MSD}\right\}-3 \mathrm{MSD}$
$0.004=\left(\frac{196}{50}-3\right)\left(\frac{1}{\mathrm{~N}}\right)$
$\mathrm{N}=\frac{46}{50} \times \frac{1000}{4}=\frac{46 \times 1000}{200}=230$