This is the 667th most frequent Polish word.
łatwo
Easily.
In this sentence, 'łatwo' functions as an adverb modifying the verb 'rozwiązał', describing the manner in which the task was solved.
On rozwiązał zadanie łatwo.
He solved the task easily.
Here, 'łatwo' is an adverbial expression indicating the ease of performing the action 'zauważyć'.
To jest łatwo zauważyć.
It is easy to notice.
In this case, 'łatwo' describes the manner in which the group overcame the difficulties, modifying the verb 'przeszli'.