Question 16

Explanation

The given number sequence is:

Let's analyze the difference between consecutive terms:

Observing the pattern of differences ($2.5, \ 5, \ 10, \ 20, \ 40$), we can see that each difference is obtained by multiplying the previous difference by $2$ (a common ratio of $2$).

Therefore, the next difference after $40$ is:

Thus, the number that fills the blank is:

To verify this pattern, let's calculate the subsequent term and check if it matches the last known term ($318$). The next difference after $80$ would be $80 \times 2 = 160$.

The calculation matches the last term in the problem.

Therefore, the correct number to complete the pattern is $158$.