Four successive members of the first series of the transition metals are listed below. For which one of them is the standard electrode potential $(E^o_{M^{2+}/M})$ value positive?

$\begin{aligned} & A(\alpha, \beta)=\left[\begin{array}{ccc}\cos \alpha & \sin \alpha & 0 \\ -\sin \alpha & \cos \alpha & 0 \\ 0 & 0 & e^\beta\end{array}\right] \\ & \Rightarrow[A(\alpha, \beta)]^{-1}=\end{aligned}$

$A$ and $B$ are two substances undergoing radioactive decay in a container. The half-life of $A$ is $15 \ min$ and that of $B$ is $5 \ min$. If the initial concentration of $B$ is $4$ times that of $A$ and they both start decaying at the same time,how much time will it take for the concentration of both of them to be same? $.......... \ min$

An electrolytic cell contains a solution of $Ag_2SO_4$ and has platinum electrodes. $A$ current is passed until $1.6 \ g$ of $O_2$ has been liberated at the anode. The amount of silver deposited at the cathode would be ............. $g$.

The equation $\sqrt{3} \sin x + \cos x = 4$ has

$Fe$ is made passive by