Count the total number of triangles of any size.
Extend the sequence.
Did you get 1, 5, 13, 27 so far?
How many triangles when there are five levels of small triangles? 10 levels?
How do you verify your sequence?
Is there a general formula for n levels?
Recognition of the sequence of triangular numbers is useful.
Count the number of triangles of each size in two parts: those pointing up and those pointing down.