Question

find the external diameter of a coin using the vernier callipers

Answer 1

Fit in the coin in the lower jaw of the vernier caliper.
keeping the coin fixed in its position -
find out the main scale reading (the reading of the main scale just before the lowest reading of the vernier scale )
find out the vernier scale reading (the reading of the vernier scale coinciding accurately with the reading of main scale )
add the two values and multiply the resultant value with the vernier constant (0.01)
the result is the external diameter of the coin.