problem 38 page 10:    some explanation

In a similar way in a 3 dimensional system the single grain (densely packed) take less volume than (2r)³ = 8r³ (a cube with side 2r). From similar (as above) considerations the volume taken by a single grain with radius r in densely packed grains is 4.2^{0.5 .}r³ - less than 8r³. To find number of grains in one cubic meter divide volume of 1m³ by the volume taken by a single grain (in densely packed sand). 

As the mass of 1 m³ is 2600kg then the mass of one grain is: grams