If a solid metal cylinder of radius 4 cm and height 10 cm is melted to make solid balls of radius 1 mm then number of such balls produced (in lakhs) will be.

Answer 1

1 mm = 10^-1 cm
pi R^2 h = n (4/3)pi r^3
so
n = R^2 h / r^3 = (3/4) 4^2 * 10 / 10^-3
=12*10^4 = 120,000

Answer 2

I am not certain about the "lakhs" part... but
You take the total volume of the original cylinder (pi)(r^2)(h) and then divide it by the volume of each of the spheres (and make sure you have the sphere of the ball as having a radius of .1cm) so your units all match up.
Volume of sphere is (4/3)(pi)(r^3)
Your answer is very rounded....
If this is a business for money then the company would like to know exactly how many such spheres could be produced : )
Or perhaps that is the lakhs part....