Four resistors of equal resistance R each are connected in various possible ways, both in series and/or parallel, to get different combined resistances. Which of the following values of resistance cannot be achieved by using all four resistors?
Show answer & explanation
Correct answer: A
Explanation
The correct answer is Option 1 i.e. 2R.
For four equal resistors of resistance R each, various series–parallel combinations give specific equivalent resistances like 4R, 2R, R, R/4, 4R/3 (≈1.33R), and 3R/4. Values such as 2R occur when two resistors in series are connected in parallel with another similar pair. 0.25R (R/4) is possible when all are in parallel, and 1.33R (4R/3) is possible from a suitable mix. However, 2.5R cannot be achieved with four identical resistors in any arrangement, making it the correct answer.