OMCPLP 2011 Problem 3

Consider a sequence of equilateral triangles $T_{n}$ as represented below:

The length of the side of the smallest triangles is $1$ . A triangle is called a delta if its vertex is at the top; for example, there are $10$ deltas in $T_{3}$ . A delta is said to be perfect if the length of its side is even. How many perfect deltas are there in $T_{20}$ ?