When 235 92U decays to 235 92U, the mass number decreases from 238 to 206 i.e. it decreases by 32. Since emission of beta particle (β) does not change the mass number and with the emission of one a (alpha) particle, mass number decreases 4, so total number of alpha particles emitted will be 32/4 = 8.
In the decay of 235 92U to lead 206 82Pb,the atomic number has decreased by 10. But due to emission of 8 αparticles, the atomic mass would have decreased by 2 x 8 = 16. Thus there is an increase in atomic number by 6, hence 6 Beta (β) particles will be emitted. (Because in emission of one beta particle, atomic number increases by 1). Thus 235 92U decays to 235 92U with emission of 8- α particles and 6 β-particles.