Shashi can do piece of work in 20 days. Tanya is 25% more efficient than Sashi. The number of days taken by Tanya to do the same piece of work is = ?

$$\eqalign{ & 25\% = \frac{1}{4}\left[ {{\text{efficiency}} \propto \frac{1}{{{\text{days}}}}} \right] \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{Sashi }}:{\text{ Tanya}} \cr & {\text{Efficiency}}\,\,\,\,\,\,\,4{\text{ }}:{\text{ }}5 \cr & {\text{Days}}\,\,\,\,\,\,\,\,\,\,\,\mathop {\mathop 5\limits_{{\text{ }} \downarrow \times 4} }\limits_{20} :\mathop {\mathop 4\limits_{{\text{ }} \downarrow \times 4} }\limits_{{\bf{16}}} \cr} $$