Suppose we have $n$ objects among which $n_1$ objects are identical, $n_2$ objects are identical, and so on (ending with $n_k$ objects being identical). Hence we have
$$n_1 + n_2 + \cdots + n_k = n$$
Now what are the formulas for $_nP_r$ and $\binom nr$ in this context? ($_nP_r$ refers to the number of permutations of $n$ objects by taking $r$ objects at a time.)