A 2 meter long scale with a least count of 0.2 cm is used to measure the locations of objects on an optical bench. While measuring the focal length of a convex lens, the object pin and the convex lens are placed at the 80 cm mark and 100 cm mark, respectively. The image of the object pin on the other side of the lens coincides with an image pin kept at the 180 cm mark. The percentage error in the estimation of the focal length is:

In a screw gauge, the 5th division of the circular scale coincides with the reference line when the ratchet is closed, indicating a positive zero error. There are 50 divisions on the circular scale, and the main scale moves by 0.5 mm during one complete rotation. For a particular measurement, the main scale reading is 5 mm and the 20th division of the circular scale coincides with the reference line. Calculate the true corrected reading.

The length of a rod is measured repeatedly by two persons. Person A reports the length to be 1002 ± 1 cm while person B reports the length to be 1001 ± 2 cm. It is known from a more reliable method that the actual length is 1000.1 ± 0.5 cm. Which one of the following statements is correct?