Explanation:

- We need to understand the pattern between the given number pairs.

- 21 : 54:

- Multiply 21 by 2.5: $21\times 2.5=52.5$.

- Add 1.5 to get 54: $52.5+1.5=54$.

- 29 : 78:

- Multiply 29 by 2.5: $29\times 2.5=72.5$.

- Add 5.5 to get 78: $72.5+5.5=78$.

- 24 : ?:

- Multiply 24 by 2.5: $24\times 2.5=60$.

- Add 3 to get the answer 63: $60+3=63$.

- Option 1: 63 is the correct answer.

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