don`t mention it 用在你帮助别人后,别人向你致谢的时候说的话,例：
A：Thank you for lending me the money!I really appreciate it!
B:don`t mention it.I am glad I can help you
no problem就是指别人需要你的帮助,你答应别人
A：Can you give me a hand to move the chair
B:no problem!My pleasure.