Complete the series 18, 40, 70, 108, _______

Let's find the differences between the consecutive numbers:
$$40-18=22$$

$$70-40=30$$

$$108-70=38$$

Now, observe the differences: $22,30,38$. The difference between these consecutive differences is constant:

$$30-22=8$$

$$38-30=8$$

Following the same logic, the next difference should be:

$$38+8=46$$

Therefore, the next number in the series is:

$$108+46=154$$