Select the number that can replace the question mark (?) in the following series.
$$3,6,12,21,33,48,?$$

Let's check the differences between consecutive terms in the series:$6-3=3$$12-6=6$$21-12=9$$33-21=12$$48-33=15$ The differences form an arithmetic sequence increasing by multiples of 3 ($3,6,9,12,\mathrm{15...}$). Following this trend, the next difference should be $18$.
$$\text{Next term}=48+18=66$$