The value of 49C3 + 48C3 + 47C3 + 46C3 + 45C3 + 45C4 is:
Consider an A.P. $a_1,a_2,\ldots,a_n$; $a_1>0$. If $a_2-a_1=-\dfrac{3}{4}$, $a_n=\dfrac{1}{4}a_1$, and \[ \sum_{i=1}^{n} a_i=\frac{525}{2}, \] then $\sum_{i=1}^{17} a_i$ is equal to

The method of forming subsets by selecting data from a larger set in a way that the selection order does not matter is called the combination.
But you are only allowed to pick three.
It is used for a group of data (where the order of data doesn’t matter).