Prepositions together? - at and around

Hi. I sometimes get confused as to whether I could use what looks to be two prepositions together, one next to the other structurally, in one sentence.

It starts around 10 a.m.

I think this is incorrect.

It starts at around 10 a.m.

I think this is correct.

It starts at around 30 dollars.

Also I think what we have below is two prepositions together (in terms of one being next to the other structually) and it is correct.

They are recruited from among those who have been proven qualified.