Units and Measurements Part 1

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