In a radioactive series, ${}_{92}{}^{238}\mathrm{U}$to ${}_{82}{}^{206}\mathrm{Pb}$ through  ${\mathrm{n}}_1$ $\mathrm{\alpha }$ decay and ${\mathrm{n}}_2 {\mathrm{\beta }}^{-} \mathrm{decay}$ processes, then

mass no decreases due to $\alpha$ decay  here mass no decreases by 238-206=32 so no of $\alpha$ particles decay 32/4=8 but along with it atomic no decreases by 8x2 =16 but here atomic no decreases 92-82=10 so to increase atomic no by 6   6 $\beta$particles has to be emitted.