17) A line of 1 cm is drawn on the drawing sheet to represent an actual length of 1 km. The RF of the scale is

18) The least count of backward vernier scale is calculated as

19) Length of a diagonal scale of RF 1:20 showing divisions of 0.01 m and capable of measuring 3 m is

20) For a bolt of diameter d and length > 150 mm, its thread length is calculated as

21) The type of rivet mostly used in shipbuilding is