Three equal resistances when combined in series are equivalent to 90 ohm. Their equivalent resistance when combined in parallel will be:

When three equal resistors are connected in series, the equivalent resistance is the sum of their individual resistances [2]. Given the series total is 90 ohm, each individual resistor (R) is calculated as 3R = 90, resulting in R = 30 ohm. For a parallel combination of these three identical resistors, the reciprocal of the equivalent resistance is the sum of the reciprocals of the individual resistances [1]. This is expressed as 1/Rp = 1/R + 1/R + 1/R, which simplifies to Rp = R/3. Substituting the individual resistance value (30 ohm) into this formula gives Rp = 30/3 = 10 ohm. Thus, the equivalent resistance of the three 30 ohm resistors when combined in parallel is 10 ohm.

Sources

1. [2] Science , class X (NCERT 2025 ed.) > Chapter 11: Electricity > Activity 11.5 > p. 184
2. [1] Science , class X (NCERT 2025 ed.) > Chapter 11: Electricity > Activity 11.6 > p. 186