A good answer might be:

Triangle Numbers
RowsTriangle(Rows) Formula
1 1 base case
2 3 2 + Triangle(1)
3 6 3 + Triangle(2)
4 10 4 + Triangle(3)
5 15 5 + Triangle(4)
6 21 6 + Triangle(5)
7 28 7 + Triangle(6)

Formula for Triangle Numbers

The table shows values of the Triangle() function. For example, Triangle(4) = 10. But it stops at seven. What is Triangle(12)?

We have already figured out that the following is true: 

total number of pins in N rows = number of pins in row N + 
                                 total number of pins in N-1 rows
                           
                               = N +
                                 total number of pins in N-1 rows

This can be written as a formula: 

Triangle( N ) = N + Triangle( N - 1 )

So the number of pins in a 12 row arrangement is 12 + number of pins in 11 rows.


QUESTION 4:

How many pins are in 11 rows?