$1 \; VSD =\frac{49}{50} \; MSD$

Least count $=1 MSD -1 VSD$

$=\left(1-\frac{49}{50}\right) MSD =\frac{1}{50} \; MSD$

$1 \; MSD =\frac{1}{40} \; cm$

Least count $=\frac{1}{50 \times 40} \; cm$

$=\frac{1}{2000} cm =\frac{1}{2} \times 10^{-5} \; m$

$=0.5 \times 10^{-5} \; m$

$=5 \times 10^{-6} \; m$