Explanation

(b) From the lens formula
$\frac{1}{f} = \frac{1}{v} - \frac{1}{u}\ we\ have,$
$\frac{1}{f} = \frac{1}{10} - \frac{1}{- 10}$
$or\ f = \ + 5$
Further, $\mathrm{\Delta}u = 0.1$
And $\mathrm{\Delta}v = 0.1$(from the graph)
Now, differentiating the lens formula we have,
$\frac{\mathrm{\Delta}f}{f^{2}} = \frac{\mathrm{\Delta}v}{v^{2}} + \frac{\mathrm{\Delta}u}{u^{2}}$
$or\ \ \mathrm{\Delta}f = \left( \frac{\mathrm{\Delta}v}{v^{2}} + \frac{\mathrm{\Delta}u}{u^{2}} \right)f^{2}$
Substituting the values we have
$\mathrm{\Delta}f = \left( \frac{0.1}{10^{2}} + \frac{0.1}{10^{2}} \right)(5)^{2} = 0.05$
$\therefore f \pm \mathrm{\Delta}f = 5 \pm 0.05$