11 plus the number of pins in 10 rows.

This keeps on going. Each round divides the problem into an integer and a problem that is smaller by one. Eventually you reach the end:

number of pins in 12 rows = 12 + number of pins in 11 rows number of pins in 11 rows = 11 + number of pins in 10 rows . . . number of pins in 3 rows = 3 + number of pins in 2 rows number of pins in 2 rows = 2 +number of pins in 1 row= 2 +1

The *number of pins in 1 row* is called a **base case**.
A *base case* is a problem that can solved immediately.
In this example, the number of pins in 1 row is 1.