Number of ways of selecting N items with K types where each type must have $n_i$ items where $\sum_i n_i = N$ So let's suppose we have $N$ items. We have $K$ types of item. 
For each type of item we denote the number of that type of item as  $n_i$ and for completeness $(1 <= i <= K)$ with the constraint that 
$\sum_i n_i = N$
How many combinations are there of this? 
So the simplest case is where $K =1$ where clearly the number of combinations is $1$. 
With $K=2$, you take $N\choose{n_1}$ or $N\choose{n_2}$ because the number of combinations of $n_1$ items in N items is the same as the number of combinations of $N-n_1$ items in $N$ items.
However, I come across a problem when I get to $K>2$. I don't know what to make of it at all and find myself very stuck at this. With $K=3$, $N=3$ and $n_i = 1 ,~\forall n_i$ then we end up with, if we label our types of item A, B and C
ABC, ACB, BAC, BCA, CAB, CBA 
Now from this I observe that for A, I placed it in a set position, of which there are N positions and then went through the combinations of the rest of it which comes out as 
${{N}\choose{n_1}} {{N-1}\choose{n_2}} = 3\times 2 = 6$ combinations
Now doing the same again but with $K=3$,  $N=4$, $n_1 = 2$, $n_2 = 1$, $n_3 =1$ we get
AABC
AACB
ACBA
ABCA
BCAA
CBAA
BAAC
CAAB
ABAC
ACAB
BACA
CABA
and the way I have done it this time is to place the two A's in set positions and then use the combinations of the others: so
${N\choose{2}} {{N-2}\choose{1}} = 6 \times 2 = 12$ combinations
So can this generalise for say $n_1 = P, ~n_2 = Q, ~n_3 = R$ with $N=P+Q+R$:
${{N}\choose{n_1}}{{N-n_1}\choose{n_2}}{{N-n_1-n_2}\choose{n_3}}$
and does that generalise to $K$ types by:
$\Pi_k{{{N-\sum_{i=0}^{i=k-1}n_i}\choose{n_k}}}$ where $n_0 = 0$?
I'm not really equipped with the skills to prove this, as I did not focus on combinatorics when I did my maths degree I did pure maths and mechanics/fluid mechanics instead, but that's my best effort and I haven't found an answer elsewhere
Thank you for listening 

Short answer: yes.
Longer answer: these are multinomial coefficients. You can express them more compactly as $$\frac{N!}{\prod n_i!}$$ which shows their origin: you can rearrange the $N$ items in $N!$ ways, but then for each $i$ you divide by $n_i!$ because the order of items of the same type doesn't matter.
