句中的 in an afternoon 用得完全正确。
I can do all this work in an afternoon.的意思是:我可以用一个下午的时间做完所有这些工作。
又如:
You deserve an afternoon off. 你应该休息一个下午。
He promised to take me to the match with the added bonus of an afternoon off school. 他答应带我去看比赛,还可以请假一个下午不用上学。
I lost half a morning’s work when the program crashed. 程序瘫痪了,我半个上午的工作都白费了。
An evening at home was a real luxury. 晚上在家里真是一种不可多得的享受。
I spent an evening out with the usual crowd. 我和经常在一起的一帮朋友在外面过了一个晚上。