A can do a work in 10 days. The efficiency of A is 20% less than B. How many days B need to finish the same work?

$$\eqalign{ & 20\% = \frac{1}{5}\left[ {{\text{efficiency}} \propto \frac{1}{{{\text{days}}}}} \right] \cr & \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{\text{A }}:{\text{ B}} \cr & {\text{Efficiency}}\,\,\,\,\,\,4{\text{ }}:{\text{ }}5 \cr & {\text{Days}}\,\,\,\,\,\,\,\,\,\,\mathop {\mathop 5\limits_{{\text{ }} \downarrow \times 2} }\limits_{10\,{\text{days}}} :\mathop {\mathop 4\limits_{{\text{ }} \downarrow \times 2} }\limits_{{\bf{8}}\,{\bf{days}}} \cr} $$