<em>Todos los pasajeros </em><em>llegaban</em><em> al mismo tiempo</em>
The original sentence is given in the simple present tense. So llegan is the conjugation of the verb llegar that matches the third person plural for this tense. To change this sentence into the imperfect, we need to conjugate the verb llegar for the third person plural in the imperfect, which is llegaban. Finally, this sentence translates into:
<em>All the passengers </em><em>arrived</em><em> at the same time</em>