Given :
The ball of a ballpoint pen is 1 mm in diameter and has an ASTM grain size of 10.
To Find :
How many grains are there in the ball.
Solution :
Volume of ball is :

Grains per unit volume is given by :
( Here, n is ASTM grain size ).
Putting n = 10 , we get :

Number of grains are :

Hence, this is the required solution.