Question

A box contains a collection of triangular and square tiles. There are 25 tiles in the box, containing 84 edges total. How many square tiles are there in the box?

Answer 1

There are 25 tiles total - triangles or squares - 84 edges
 
triangle (t) = 3 sides
square (s) = 4 sides

t+s= total tiles 
t+s = 25 ← solve
3t + 4s = total sides
3t + 4s = 84 ← solve

t = 16   s = 9

Check

16 + 9 = 25

3(16) + 4(9) = 84
48 + 36 = 84

There are 9 square tiles in the box