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The question asks about a sequence of 11 consecutive pages Ria read, and the sum of those pages is 209. We need to find the last page number she read.

### Solution

Let:
- The first page Ria read be \( x \).
- Since she read 11 consecutive pages, the pages are \( x, x+1, x+2, \dots, x+10 \).

The sum of these pages can be expressed as:
\[
x + (x+1) + (x+2) + \dots + (x+10) = 209
\]

This is an arithmetic sequence with:
- First term \( a = x \)
- Number of terms \( n = 11 \)
- Last term \( l = x + 10 \)

The sum \( S \) of an arithmetic sequence can be given by:
\[
S = \frac{n}{2} \times (a + l)
\]
Substituting the values, we get:
\[
209 = \frac{11}{2} \times (x + (x + 10))
\]
Simplifying,
\[
209 = \frac{11}{2} \times (2x + 10)
\]
\[
209 = 11(x + 5)
\]
\[
x + 5 = 19
\]
\[
x = 14
\]

So, the first page Ria read was page 14, and the last page was:
\[
x + 10 = 14 + 10 = 24
\]

### Answer:
The last page number Ria read is **B. 24**.

Would you like more details on the solution or have any other questions?

### Related Questions:
1. How would the solution change if Ria read 15 pages instead of 11?
2. What would be the first page number if the sum was 300 instead of 209?
3. How is the sum formula derived for an arithmetic sequence?
4. Can this approach be used if the pages are not consecutive but still form a sequence?
5. How would the answer change if the pages were counted in descending order?

**Tip:** When solving problems involving consecutive numbers, the arithmetic sequence formula simplifies calculations.

Ria membaca 11 halaman buku berurutan. Jumlah 11 halaman yang ia baca adalah 209. Nomor halaman terakhir yang Ria baca adalah?

Determine the last page number Ria read in a sequence of 11 consecutive pages with a total sum of 209. This problem covers arithmetic sequences and the sum formula for consecutive integers, suitable for students in Grades 8-10.

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