Two numbers are respectively 20% and 50% of a third number. What per cent is first number of second?

$$\eqalign{ & {\text{Let third number is x}}. \cr & {\text{Then}}\,{\text{first}}\,{\text{no}}{\text{.}} \cr & 20\% \,{\text{of}}\,x = \frac{{20x}}{{100}} \cr & {\text{Second}}\,{\text{number}} \cr & = 50\% \,{\text{of}}\,x = \frac{{50x}}{{100}} \cr & {\text{Percent of first no of second no,}} \cr & = {\frac{{ {\frac{{20x}}{{100}}} }}{{ {\frac{{50x}}{{100}}} }}} \times 100 \cr & = \frac{{ {2 \times 100} }}{{20}} \cr & = 40\% \cr} $$