131.Solving a chemistry problem

20. A solution of gold sol containing 50 mg / dm 3 Au is diluted in 1000
times and studied under an ultramicroscope. The total number of particles in the field of view with an area of 1⋅10-9 m 2 and a beam depth of 2,0⋅10-5 m is equal to 65. Assuming that the gold particles are spherical and have the same density as solid gold, ρ = 19,3⋅10 3 kg / m 3, calculate their average radius.