139. $16^{x}=1 / 4$.

139. $16^{x}=1 / 4$. 

(Note: The translation is the same as the original text because it is a mathematical equation, which is universal and does not change in translation.)
Solution. Since $16=2^{4}, 1 / 4=2^{-2}$, the equation will take the form $2^{4 x}=2^{-2}$, from which $4 x=-2$, i.e., $x=-1 / 2$.