for a real image 3 times the size of the object, it implies that the magnification 3 = v/u, 3 = 24/u, therefore 3u = 24, u = 8cm thus we have from the relation:m = (f)/(u)-f; that 3 = (f)/(8) - f. therefore f = 3(8-f) f = 24 - 3f ∴4f = 24; f = 6cm OR: (1)/(u) + (1)/(v) = (1)/(f). (1)/(8) + (1)/(24) = (1)/(f) therefore (3+1)/(24) = (1)/(f) 4/24=1/f;therefore 4f = 24 f = 6cm