Which of the following numbers will replace the question marks (?) in the given series?
$$1,6,15,28,?,?,91$$

Let's discover the hidden pattern by computing the differences between the consecutive numbers given in the sequence:
* $6-1=5$
* $15-6=9$
* $28-15=13$

The differences between successive terms ($5,9,13$) increase progressively by $+4$ each time (forming an Arithmetic Progression).

Continuing this pattern of differences:

* Next difference should be $13+4=17$.

$$\text{First missing term}=28+17=45$$

* The following difference should be $17+4=21$.

$$\text{Second missing term}=45+21=66$$

Let's verify if it matches the last term:

* The next difference should be $21+4=25$.

$$66+25=91(\text{Matches the last term perfectly})$$

Thus, the missing values are $45$ and $66$.