Let's identify the pattern in the sequence:

The sequence is: 3, 6, 9, 15, 24, 39.

We can find the difference between consecutive terms:

• $6 - 3 = 3$
• $9 - 6 = 3$
• $15 - 9 = 6$
• $24 - 15 = 9$
• $39 - 24 = 15$

The differences between the terms form this sequence: 3, 3, 6, 9, 15.

Now, observe that the differences are increasing as follows:

• $3, 6, 9, 15$, which suggests that the next difference could be $15 + 6 = 21$.

Therefore, to find the next number, we add 21 to the last term in the sequence:

$39 + 21 = 60$

Thus, the next number in the sequence is 60.

Would you like more details or have any questions?

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Here are 5 related questions to explore this further:

1. What would the 8th term in this sequence be?
2. Can you find a formula to express the $n$-th term in this sequence?
3. What kind of sequences have non-constant differences?
4. How would you extend this pattern to find the 20th term?
5. How does this sequence differ from an arithmetic progression?

Tip: When differences between terms form a sequence themselves, this may suggest an additive or recursive pattern.