Solution :
Given :$\tan \theta = \large\frac{20}{21} $
Dividing all terms of $\large\frac{\cos \theta - \sin \theta }{\cos \theta +\sin \theta}$ by $\cos \theta$
$\qquad=\large\frac{1- \tan \theta}{1+\tan \theta }$
$\qquad= \large\frac{1- \Large\frac{20}{21}}{1+\Large\frac{20}{21}}$
$\qquad= \large\frac{21-20}{21+20}$
$\qquad= \large\frac{1}{41}$