N3
当たり
あたり
Per~, A~, Apiece, Each
これは1個当たり50キロカロリーあります。
These have 50 kilocalories apiece.
Explanation
「当たり」 follows a basic unit such as time, number of people, area or capacity, and means 'per...'. With that unit as the base, it gives the amount shared or used per unit. It is used in objective calculation such as statistics, costs, wages and efficiency: 「1人当たりの費用は5,000円です」 (The cost per person is 5,000 yen); 「1日当たり3時間勉強する」 (I study three hours per day).
Mixed up with
「当たり」 suits everyday money and averages. 「につき」 is more formal and official, and in writing can also mean a cause (due to construction...).
「当たり」 stresses the relation between unit and rate (per day...). 「ずつ」 stresses the actual share handed out (two copies each).