~~I'm editing in a note at the top of my response here, because defining row and column in the way that you have leads to a different sort of problem than what I have addressed. Apologies~~
~~The value you are looking for is (C+R-2)choose(R-1) where C and R are the row and column values in your examples.~~
The triangle diagram you have included is incorrectly labelled. The rows do not increase on the RHS in the same way that the columns increase on the LHS. Consider that the first row is the 0th row and the first column is the 0th column. So the 0th row has only 1 column. The 1st row has only 2 columns. The nth row has n+1 columns (because the first column of each row is the 0th column). That is, the rows go horizontally (starting from the top) and the columns go vertically (starting from the left); they are not angled as is shown in your diagram.
In your first example, the 3rd element on the 4th row has a value of 4, counting the row and column from 0. Using natural numbers, this would appear to be the 4th element of the 5th row.
In your second example, the 3rd row does not have a 5th column, and so the result of 15 is incorrect.
To answer your question, the best way to solve for any location in the triangle is by drawing it out or by calculating n!/r!(n-r)! Keep in mind that the row and column start at (0,0) and also that the triangle is horizontally symmetrical.
Here is my source for this answer: https://en.wikipedia.org/wiki/Binomial_coefficient