Question

the question is in the above attachment with formula​

Answer 1

10ohm + 20ohm=30ohm

5ohm + 25ohm= 30 ohm

1/R= 1/r1 + 1/r2

1/R=1/30 + 1/30

1/R=2/30

R=15ohm

Answer 2

Answer :

✴ Eq. resistance of series :

$\bigstar\:\boxed{\tt{R=R_1+R_2}}$

✴ Eq. resistance of parallel :

$\bigstar\:\boxed{\tt{\dfrac{1}{R}=\dfrac{1}{R_1}+\dfrac{1}{R_2}}}$

➳ As per ohm's law, current flow through a conductor is directly proportional to the applied potential difference.

Mathematically, V = I × R

✪ Eq. of upper part (series) :

$\longrightarrow\tt\:R=R_1+R_2\\ \\ \longrightarrow\tt\:R=10+20\\ \\ \longrightarrow\tt\:R=30\Omega$

✪ Eq. of lower part (series) :

$\longrightarrow\tt\:R'=R_3+R_4\\ \\ \longrightarrow\tt\:R'=5+25\\ \\ \longrightarrow\tt\:R'=30\Omega$

✪ Net eq. of circuit (parallel) :

$\longrightarrow\tt\:\dfrac{1}{R_{eq}}=\dfrac{1}{R_1}+\dfrac{1}{R_2}\\ \\ \longrightarrow\tt\:\dfrac{1}{R_{eq}}=\dfrac{1}{30}+\dfrac{1}{30}\\ \\ \longrightarrow\tt\:\dfrac{1}{R_{eq}}=\dfrac{2}{30}\\ \\ \longrightarrow\underline{\boxed{\bf{R_{eq}=15\Omega}}}$

✪ Current flow :

$\longrightarrow\tt\:V=I\times R_{eq}\\ \\ \longrightarrow\tt\:12=I\times 15\\ \\ \longrightarrow\underline{\boxed{\bf{I=0.8A}}}$