When 235 92U decays to 206 82Pb, the mass number decreases from 238 to 206 i.e., it decreases by 32. Since with the emission of one alpha particle, mass number decreases by 4, so total number of alpha particles emitted will be 32/4 = 8
In decay of 235 92U to 206 82Pb, the atomic number has decreased by 10. But due to emission of 8 alpha particles, the atomic number would have decreased by 2 x 8 = 16. Thus there is an increase in atomic number by 6, hence 6 beta particles will be emitted (because in emission of one beta particle, atomic number increases by 1). Thus 235 92U decays to 206 82Pb with the emission of 8 a-particles and 6 P-particles.
