Question

the question is given below​

Answer 1

For series: R1 + R2 +.... Rn.

For parallel: 1/R1 + 1/R2 +.... 1/Rn.

Here 2 ohms + 2 ohms (series) = 4 ohms.

Now, that's in parallel with 1 ohm.

Formula = R1R2 / R1 + R2 = 4 * 1 / 4 + 1 = 4/5.