This is the 931st most frequent Croatian word.
prosjeku
"Prosjeku" in English means "average."
Here, 'prosjek' (noun nominative singular) is used to refer to the mathematical average of his grades.
Njegov prosjek ocjena je iznad očekivanog.
His grade average is above expectations.
In this sentence, 'prosjeku' (dative singular of 'prosjek'), is used to mean 'on average' to indicate a general tendency or statistic.
U prosjeku, prosječan student troši tri sata dnevno na učenje.
On average, an average student spends three hours per day studying.
Again, 'prosjeku' (dative singular of 'prosjek') is used as 'on average' to indicate general climatic tendencies.