Question: the focal length of a concave mirror is 50 cm where an object?
Answer: By making the image distance twice the object distance ( i = 2 o), then : 1/f = 1/o + 1/(2o) = 2/2o + 1/2o = 3/(2o), so f = 2o/3 or o = 1.5f. that makes object distance 75cm.