Question

find the solution when the key is opened and when the key is closed

Answer 1

if key is open there will be no flow of current .
but when the key is closed the current will divided by four resistor by dividing 3 ohm = 2ampere per resistor . the final answer is 1.5 .
hope this will help you

Answer 2

I = V/R

V= 12v

when key is closed:

R=

R1 and R2 are in series, Rs = 4+4 = 8 ohm

R3 and R4 are in series , Rs = 4+4= 8 ohm

Rs and Rs are in parallel combination

1/Rp = 1/8+1/8

=2/8

=1/4

Rp= 4 ohm

I=V/R

=12/4

=3 A

when key is closed:

current will not pass through R3 and R4

Rs= 8 ohm

I=V/R

=12/8

=3/2

=1.5 A