Decode the secret message using the number key in this fun math and literacy activity.

Problem Description:

The task involves decoding a secret message using a provided key. The key maps numbers to letters, and the encoded message consists of a series of numbers. The goal is to decode the message step by step.

Key Provided:

The key is a mapping of numbers to letters as follows:

| Number | Letter |
|--------|--------|
| 1 | A |
| 2 | B |
| 3 | C |
| 4 | D |
| 5 | E |
| 6 | F |
| 7 | G |
| 8 | H |
| 9 | I |
| 10 | J |
| 11 | K |
| 12 | L |
| 13 | M |
| 14 | N |
| 15 | O |
| 16 | P |
| 17 | Q |
| 18 | R |
| 19 | S |
| 20 | T |
| 21 | U |
| 22 | V |
| 23 | W |
| 24 | X |
| 25 | Y |
| 26 | Z |

Encoded Message:

The encoded message is given as a sequence of numbers:
```
19 20 1 25 3 15 15 12 9 14
20 8 5 4 15 7 4 1 25 19
15 6 19 21 13 13 5 18
```

Solution Approach:

1. Understand the Key: Each number in the encoded message corresponds to a specific letter based on the key.
2. Decode Each Number: Replace each number in the encoded message with its corresponding letter.
3. Form the Decoded Message: Combine the letters to form the final decoded message.

Step-by-Step Decoding:

#### First Row: `19 20 1 25 3 15 15 12 9 14`
- 19 → S
- 20 → T
- 1 → A
- 25 → Y
- 3 → C
- 15 → O
- 15 → O
- 12 → L
- 9 → I
- 14 → N

Decoded: STAY COOLIN

#### Second Row: `20 8 5 4 15 7 4 1 25 19`
- 20 → T
- 8 → H
- 5 → E
- 4 → D
- 15 → O
- 7 → G
- 4 → D
- 1 → A
- 25 → Y
- 19 → S

Decoded: THE DOG DAYS

#### Third Row: `15 6 19 21 13 13 5 18`
- 15 → O
- 6 → F
- 19 → S
- 21 → U
- 13 → M
- 13 → M
- 5 → E
- 18 → R

Decoded: OFFSUMMER

Final Decoded Message:

Combining all the rows, the decoded message is:
```
STAY COOLIN
THE DOG DAYS
OFFSUMMER
```

Explanation:

- The first row suggests staying cool.
- The second row indicates that it's the dog days (a period of hot weather).
- The third row implies that summer is off (over).

Final Answer:

$$
\boxed{\text{STAY COOLIN THE DOG DAYS OFFSUMMER}}
$$