Dear Mr Wang,
I'm writing you this letter to apologize for my failing to hand in the paper on time. I'm very sorry for the convenience brought by my delay. Please accept my sincere apology with gratitude. The reason for my delay was the time was knocked down by a cold. Moreover,I had no way out because I was too weak to write a single word. Therefore,it is not in my power to finish the paper on time. I hope you will understand me and excuse me for my delay. However, I would do my utmost to finish the paper and hand it in as soon as possible. In my opionion,the importance of punctuality cannot be emphasized too much. So I assure you that I will hand in my homework on time from now on.