a) From the observations, it is clear that for u = 30, v = 30 cm. This means this value must be equal to twice the focal length of the convex lens.
∴ Focal length of convex lens = 30/2 = 15 cm
b) The observation (5) is not correct because if u = 15 cm i.e., the object is kept at focus so the image should be at infinity and not at 70 cm.
c) For twice the focal length we know size of object = size of image. So when object is kept at 30 cm the size of object and image are same.