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The problem given is:

- A boy is 3 years old, and his sister is three times as old as he is. 
- When the boy is 12 years old, how old will his sister be?

### Step-by-step solution:

1. **Current Ages:**
   - The boy is currently 3 years old.
   - His sister is 3 times older than him, so her current age is \( 3 \times 3 = 9 \) years.

2. **Future Ages:**
   - The question asks about their ages when the boy is 12 years old.
   - The boy will be 12 years old, which is 9 years older than he is now (since \( 12 - 3 = 9 \)).

3. **Sister's Age in the Future:**
   - Since 9 years will pass for both of them, we add 9 years to the sister's current age:
   \[
   9 + 9 = 18
   \]
   Therefore, when the boy is 12 years old, his sister will be 18 years old.

### The correct answer is (a) 18.

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

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Here are five related questions you might consider:
1. If the boy is currently 5 years old, how old is his sister, assuming the same age relationship?
2. How many years older is the sister compared to the boy?
3. If the boy was 10 years old and his sister was twice as old as him, how old is his sister now?
4. What will be the age difference between the siblings at any point in the future?
5. If the boy turns 15 years old, how old will his sister be based on the current scenario?

**Tip:** When dealing with age problems, always focus on the difference in age, as it remains constant over time!

A boy is 3 years old, and his sister is three times as old as he is. When the boy is 12 years old, how old will his sister be?

Solve a classic age problem where a boy is currently 3 years old, and his sister is three times his age. Find out how old his sister will be when the boy turns 12, focusing on age differences and ratios. Suitable for Grades 5-7.

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