Distributing k objects among m people where each person gets i objects such that aIn an examination, the score in each of the four subjects ( say )  - A ,B,C & D can be between integers 0 and 10. Then , how many are there such that the student can secure a total of 21 ?
My attempt - My initial attempt was to find out the total number of ways in which the student could score 21 and then look to subtract the number of ways which included a student scoring above 10. But I realised that this would include a lot of cases to be considered as I will have to weed out manually the attempts for each integer great than 10 which is too computationally intensive . 
Please tell me if there is any formulae ( along with justification for the formulae ) for solving the same.

Let $a$, $b$, $c$, $d$ denote the student's scores in subjects $A$, $B$, $C$, and $D$, respectively.  Then 
$$a + b + c + d = 21 \tag{1}$$
is an equation in the non-negative integers subject to the constraints $a, b, c, d \leq 10$.  
A particular solution of equation 1 corresponds to the insertion of three addition signs in a row of $21$ ones.  For instance,
$$1 1 1 1 1 1 + 1 1 1 1 1 1 1 + 1 1 + 1 1 1 1 1 1$$
corresponds to $a = 6$, $b = 7$, $c = 2$, and $d = 6$, while 
$$1 1 1 1 1 1 1 + 1 1 1 1 1 1 + + 1 1 1 1 1 1 1 1$$
corresponds to $a = 7$, $b = 6$, $c = 0$, and $d = 8$.  Thus, the number of solutions of equation 1 is the number of ways three addition signs can be inserted into a row of $21$ ones, which is 
$$\binom{21 + 3}{3} = \binom{24}{3}$$
since we must select which three of the $21$ symbols (three addition signs and $21$ ones) will be addition signs.
However, we must exclude solutions in which one or more of the variables exceeds $10$.  Observe that at most one of the variables can exceed $10$ since $2 \cdot 11 = 22 > 21$.  
Suppose $a > 10$.  Let $a' = a - 11$.  Then $a'$ is a non-negative integer.  Substituting $a' + 11$ for $a$ in equation 1 yields
\begin{align*}
a' + 11 + b + c + d & = 21\\
a' + b + c + d & = 10 \tag{2}
\end{align*}
Equation 2 is an equation in the non-negative integers with $\binom{10 + 3}{3} = \binom{13}{3}$ solutions.  By symmetry, there are also $\binom{13}{3}$ solutions in which $b$, $c$, or $d$ exceeds $10$.  Hence, there are 
$$\binom{4}{1}\binom{13}{3}$$
solutions of equation 1 in which one of the variables exceeds $10$.  
Thus, the number of solutions of equation 1 in which none of the variables exceeds $10$ is 
$$\binom{24}{3} - \binom{4}{1}\binom{13}{3}$$ 
