If the current in a coil is halved,then the energy stored becomes how many times the previous value?

$(a)$ Obtain the expression for the magnetic energy stored in a solenoid in terms of magnetic field $B$,area $A$,and length $l$ of the solenoid.
$(b)$ How does this magnetic energy compare with the electrostatic energy stored in a capacitor?

$A$ coil of resistance $10 \, \Omega$ and an inductance $5 \, H$ is connected to a $100 \, V$ battery. The energy stored in the coil is:

$A$ coil of inductance $2\, H$ having negligible resistance is connected to a source of supply whose voltage is given by $V = 3t$ volt (where $t$ is in seconds). If the voltage is applied when $t = 0$,then the energy stored in the coil after $4\, s$ is ............ $J$.

Energy required to establish a current of $4 \ A$ in a coil of self-inductance $L = 200 \ mH$ is . . . . . . . (in $J$)

Assertion : If the current in a solenoid is reversed in direction while keeping the same magnitude,the magnetic field energy stored in the solenoid remains unchanged.
Reason : Magnetic field energy density is proportional to the magnetic field.