What is the sixth number in the sequences: a) 6, 12, 18, 24, and b) 2, 4, 8, 16, ? What is the answer?

The sequence is $2, 4, 8, 16, \dots$. To find the pattern, we calculate the ratio between consecutive terms:

$$\frac{4}{2} = 2$$ $$\frac{8}{4} = 2$$ $$\frac{16}{8} = 2$$

The common ratio between terms is 2. This means it is a geometric sequence where each term is obtained by multiplying the previous term by 2.

The given terms are the first four numbers: $2, 4, 8, 16$.
To find the fifth number, multiply the fourth number by 2:

$$a_5 = 16 \times 2 = 32$$

To find the sixth number, multiply the fifth number by 2:

$$a_6 = 32 \times 2 = 64$$

The sixth number in the sequence is $\boxed{\text{64}}$.