A vernier’s zero lies between main divisions 20 and 21; its fourth division coincides with a main division. Five vernier divisions equal four main divisions, and one main division = 1 mm. Find reading (one decimal mm).
One vernier division is 4/5 mm, so least count is $1-4/5=0.2$ mm. Reading = main-scale value + coinciding-division count × least count = 20 + 4(0.2) = **20.8 mm**.
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