$u =-30 cm$

$v =60 cm$

$\& \frac{1}{ f }=\frac{1}{ v }-\frac{1}{ u }$ on solving $: f =20 cm$

also $\frac{1}{ f }=\frac{1}{ v }-\frac{1}{ u }$

on differentiation

$\frac{d f}{f^2}=\frac{d v}{v^2}+\frac{d u}{u^2}$

$\frac{d f}{f}=f\left[\frac{d v}{v^2}+\frac{d u}{u^2}\right]$

$\& \frac{d f}{f} \times 100=f\left[\frac{d v}{v^2}+\frac{d u}{u^2}\right] \times 100 \%$

$f=20 cm , d u=d v=\frac{1}{4} cm$

Since there are $4$ divisions in $1 cm$ on scale

$\therefore \frac{ df }{ f } \times 100=20\left[\frac{1 / 4}{(60)^2}+\frac{1 / 4}{(30)^2}\right] \times 100 \%$

$=5\left[\frac{1}{3600}+\frac{1}{900}\right] \times 100 \%$

$=5\left[\frac{5}{36}\right] \%=\frac{25}{36} \% \approx 0.69 \%$