A piece of wire of resistance R is cut into five equal parts. These parts are then connected in parallel. If the equivalent resistance of this combination is R', then the ratio R/R' is

The correct option is D
25
Step 1: Resistance of each part after cutting
We know that, Resistance R = ρl/A
Since, R is directly proportional to length of the wire.
Therefore the resistance of every part after cutting the wire in 5 parts is
R1 = R2 = R3 = R4 = R5 = R/5
Step 2: Equivalent Resistance
When all the resistances are connected in parallel, let R' be the equivalent resistance
So, 1/R' = 1/R1 + 1/R2 + 1/R3 + 1/R4 + 1/R5
⇒ 1/R' = 5/R + 5/R + 5/R + 5/R + 5/R
⇒ R' = R/25
⇒ R/R' = 25
Hence, Option (D) is correct.