If two numbers are respectively 20% and 50% of a third number, what is the percentage of the first number to the second?

Suppose that the third number is $x$ 

Hence the first number is $0.2x$, second number is $0.5x$

Therefore, the percentage of first number to the second is  $\frac{0.2x}{0.5x}\times 100=40\%$