Answer key for a rational numbers worksheet featuring a riddle and math problems.

To solve the problem and explain the solution, let's break it down step by step.

Step 1: Understand the Task

The task involves adding or subtracting rational numbers from a list of problems (labeled A through R). After solving each problem, you will match the answer to a letter in the "Answer Key" section. The letters will spell out the answer to the riddle: "Why did the cookie go to the doctor?"

Step 2: Solve Each Problem

We will solve each problem one by one and find the corresponding letter in the "Answer Key."

#### Problem A: -7 - 13
\[
-7 - 13 = -20
\]
Answer: -20 → Letter: B

#### Problem B: 12 - 42
\[
12 - 42 = -30
\]
Answer: -30 → Letter: E

#### Problem C: -8 - (-13)
\[
-8 - (-13) = -8 + 13 = 5
\]
Answer: 5 → Letter: C

#### Problem D: -9 - (-6)
\[
-9 - (-6) = -9 + 6 = -3
\]
Answer: -3 → Letter: A

#### Problem E: -1 - (-8)
\[
-1 - (-8) = -1 + 8 = 7
\]
Answer: 7 → Letter: U

#### Problem F: -6 + (-2)
\[
-6 + (-2) = -6 - 2 = -8
\]
Answer: -8 → Letter: S

#### Problem G: -6 + (-4)
\[
-6 + (-4) = -6 - 4 = -10
\]
Answer: -10 → Letter: E

#### Problem H: -50 ÷ (-5)
\[
-50 ÷ (-5) = 10
\]
Answer: 10 → Letter: I

#### Problem I: -2 - (-7)
\[
-2 - (-7) = -2 + 7 = 5
\]
Answer: 5 → Letter: C

#### Problem J: 0.5 - 1.5
\[
0.5 - 1.5 = -1
\]
Answer: -1 → Letter: T

#### Problem K: -0.1 - 0.7
\[
-0.1 - 0.7 = -0.8
\]
Answer: -0.8 → Letter: R

#### Problem L: -0.06 + 0.06
\[
-0.06 + 0.06 = 0
\]
Answer: 0 → Letter: Y

#### Problem M: 6 - 7.32
\[
6 - 7.32 = -1.32
\]
Answer: -1.32 → Letter: M

#### Problem N: -0.25 - 0.75
\[
-0.25 - 0.75 = -1
\]
Answer: -1 → Letter: T

#### Problem O: 2.2 - 5.1
\[
2.2 - 5.1 = -2.9
\]
Answer: -2.9 → Letter: U

#### Problem P: -32 + 5.6
\[
-32 + 5.6 = -26.4
\]
Answer: -26.4 → Letter: N

#### Problem Q: 7.31 - 8.61
\[
7.31 - 8.61 = -1.3
\]
Answer: -1.3 → Letter: A

#### Problem R: -6.8 - 21.7
\[
-6.8 - 21.7 = -28.5
\]
Answer: -28.5 → Letter: Y

Step 3: Match Answers to Letters

Now, we match each answer to the corresponding letter in the "Answer Key":

- A: -20 → B
- B: -30 → E
- C: 5 → C
- D: -3 → A
- E: 7 → U
- F: -8 → S
- G: -10 → E
- H: 10 → I
- I: 5 → C
- J: -1 → T
- K: -0.8 → R
- L: 0 → Y
- M: -1.32 → M
- N: -1 → T
- O: -2.9 → U
- P: -26.4 → N
- Q: -1.3 → A
- R: -28.5 → Y

Step 4: Spell Out the Answer

Using the letters obtained from the answers, we spell out the message:

\[
B, E, C, A, U, S, E, I, C, E, T, R, Y, M, T, U, N, A, Y
\]

Final Answer:

The answer to the riddle "Why did the cookie go to the doctor?" is:

\[
\boxed{\text{Because I Ceiling Treatment}}
\]