7. Solving a chemistry problem

270. When studying the sol by the Deryagin-Vlasenko flow ultramicroscopy method, n particles were counted in the volume of the sol Vm3, which flowed through the counting field of the microscope. Determine the average particle size as given in Table 21.
Sol V, m3 n С, kg/m3 r, kg/m3 Average r or l
Oil mist 1.33*10-11 50 25.10-6 0.9*103 Radius