I believe this question asks for the total amount of area of wood that is needed to make 80 puzzles.
The formula for area of triangle is given as:
A = (1 / 2) b h
where,
b = is the base of the triangle = 3 in
h = height of the triangle = 4 in
So calculating for area of 1 on triangle (A1):
A1 = (1 / 2) (3 in) (4 in)
A1 = (1 / 2) 12 in^2
A1 = 6 in^2
We are given that we are to make 80 puzzles and each puzzle has 20 triangles, therefore the total amount of triangles we need to make is:
n = 80 * 20
n = 1600
Therefore the total area needed (A) is:
A = A1 * n
A = 6 in^2 * 1600
A = 9,600 in^2