Four dice are rolled, then the probability that at least one digit on the dice must be repeated is

Let $\text{'}E\text{'}$  be event of getting 4 different digits

$n\left(E\right)=6\times 5\times 4\times 3$

$P\left(E\right)=\frac{6\times 5\times 4\times 3}{6\times 6\times 6\times 6}=\frac{5}{18}$

$\therefore P\left(E\right)=$  Probability of getting at least are digit is repeated is

$=1-P\left(E\right)=1-\frac{5}{18}=\frac{13}{18}$