If $f(x)=\log _e\left(\frac{1-x}{1+x}\right),|x|<1$, then $f\left(\frac{2 x}{1+x^2}\right)$ is equal to

If the half life of substance is 5 years, then the total amount of the substance left after 15 years, when initial amount is 64 gms is

If $$\log _2 x+\log _4 x+\log _8 x+\log _{16} x=\frac{25}{36}$$ and $$x=2^{\mathrm{k}}$$ then $$\mathrm{k}$$ is

The approximate value of $$\log _{10} 998$$ is (given that $$\log _{10} \mathrm{e}=0.4343$$ )