A corn cob shaped somewhat link of Kaun healthy radius of it broadest

This question was asked in Ex 13.3, 7

A corn cob shaped somewhat link of Kaun healthy radius of it broadest end as 2.1cm and length (height) as20 centimetre if each one centimetre square of the cob cerries an average of four grains,find how many grains you would find on the entire cob