How many $\alpha$ and $\beta$-particles are emitted when uranium nucleus $\left({{}_{92}\mathrm{U}}^{238}\right)$ decay to ${{}_{82}\mathrm{P}}^{214}$ ?

When an alpha particle is emitted from a radioactive substance its atomic number(Z) decreases by 2 and mass number(A) decreases by 4. When β  particle is emitted from a radioactive substance its atomic number(Z) increases by 1 and mass number(A) does not change.
Here, Uranium decays to lead its mass no changes to by 24 i.e. 

24/4=6α particles are emitted and hence, its atomic 

number changes by 12 but the atomic number of Pb​ is 10 this is due to emission of two β particle.