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I study English on average 2 hours a day.
Literal
I [topic-は] on-average one-day [per-に] 2-hours English [object-を] study.
平均して ('on average,' the te-form of 平均する 'to average') functions as an adverbial. 一日に ('per day') uses に for the 'per unit' meaning — 'one day per which [this amount applies].' A common learner statement about study habits.