Solve the fruit math puzzle: What is the value of the missing sum?

To solve this problem, we need to determine the values of the fruits (cherries, apples, and bananas) based on the given equations. Let's break it down step by step.

Step 1: Define Variables

Let:
- \( C \) represent the value of a bunch of cherries.
- \( A \) represent the value of an apple.
- \( B \) represent the value of a banana.

Step 2: Write Down the Equations

From the image, we have the following equations:

1. \( C + C + C = 15 \)
2. \( C + A + A = 23 \)
3. \( B + A + A = 21 \)
4. \( C + B + A = ? \)

Step 3: Solve for \( C \)

From the first equation:
\[
C + C + C = 15
\]
\[
3C = 15
\]
\[
C = 5
\]

Step 4: Solve for \( A \)

From the second equation:
\[
C + A + A = 23
\]
Substitute \( C = 5 \):
\[
5 + A + A = 23
\]
\[
5 + 2A = 23
\]
\[
2A = 18
\]
\[
A = 9
\]

Step 5: Solve for \( B \)

From the third equation:
\[
B + A + A = 21
\]
Substitute \( A = 9 \):
\[
B + 9 + 9 = 21
\]
\[
B + 18 = 21
\]
\[
B = 3
\]

Step 6: Solve the Final Equation

Now, we need to find the value of \( C + B + A \):
\[
C + B + A = 5 + 3 + 9
\]
\[
C + B + A = 17
\]

Final Answer:

\[
\boxed{17}
\]