Use Euler's formula to find the missing number.

Vertices: 23
Edges: 39
Face: ?

Using Euler's formula:

F + V - E = 2

where F is the number of faces, V is the number of vertices, and E is the number of edges.

Plugging in the given values:

F + 23 - 39 = 2

Simplifying:

F - 16 = 2

Adding 16 to both sides:

F = 18

Therefore, the missing number is 18.

Euler's formula states that for a polyhedron, the number of vertices (V), edges (E), and faces (F) are related by the equation: V - E + F = 2.

Given that the number of vertices is 23 (V = 23) and the number of edges is 39 (E = 39), we can substitute these values into the formula and solve for the missing number:

23 - 39 + F = 2

Begin by simplifying the equation:

-16 + F = 2

Next, isolate F by adding 16 to both sides:

F = 2 + 16

Finally, compute the sum:

F = 18

Therefore, the missing number, representing the number of faces, is 18.