To find the missing number in the last pie chart, we can analyze the pattern in the first two pie charts.

1. **First pie chart**:
- Green: 5
- Red: 6
- Blue: 15
- Total: 5 + 6 + 15 = 26

2. **Second pie chart**:
- Green: 6
- Red: 7
- Blue: 21
- Total: 6 + 7 + 21 = 34

Now we look at the totals for each:

- First chart total (26) and second chart total (34) suggest a pattern of increasing totals.

The totals appear to consistently increase by 8 (34 - 26 = 8).

For the third pie chart, we can check the given values:

- Green: 7
- Red: 10
- Let's denote the Blue value as \( x \).

Now we can set up the equation for the total based on the established pattern. Assuming the total for the third chart continues this increase from 34,

\[ 34 + 8 = 42 \]

So we calculate \( x \):

\[ 7 + 10 + x = 42 \]
\[ 17 + x = 42 \]
\[ x = 42 - 17 \]
\[ x = 25 \]

Thus, the missing number in the third pie chart is **25**.