in
Yanlış : He smokes on an average ten cigarettes in a day.
Doğru : He smokes on an average ten cigarettes a day.
Günde ortalama olarak on sigara içer.
Yanlış : His father makes him an allowance of 100 liras in a week.
Doğru : His father makes him an allowance of 100 liras a week.
Babası ona haftada yüz lira harçlık veriyor.
Sıklık, hız, süre, vb. gösteren ifadelerde in kullanılmaz.