The simplest ratio of triangles to total shapes is 1:5. This ratio can be expressed in a number of different ways, such as 1/5, 1:5, or 1/5. In this ratio, the first number represents the number of triangles and the second number represents the total number of triangles and circles. Since there are 4 triangles and 16 circles, the total number of shapes is 4 + 16 = 20. Therefore, the ratio of triangles to total shapes is 1:5, which is the simplest form of this ratio.