Series #4

The difference between 1 and 5 is 4 $=2^2$

The difference between 5 and 14 is 9 $=3^2$

The difference between 14 and 30 is 16 $=4^2$

The difference between 30 and 55 is 25 $=5^2$

$$

So the difference between 55 and ? must be $6^2$ = 36

Therefore 55 + 36 = $\boxed{91}$

$$

We can check the answer because the difference between ? and 140 should be $7^2$ = 49, and 140 - 49 = 91, as required.

The difference is in ascending square numbers