Homework Help: Lamps in series and parallel

I'm stuck on this homework I need to do but it's for tomorrow and it's very important. Could one of you please answer this? The question is:
There are 10 different combinations that lamps can be arranged if there are 5 of them and you can organise them only in series and only in parallel. The formula for calculating the total resistance in parallel is 1/Rt = 1/R1 + 1/R2 etc until 5. And for series it is Rt = R1 + R2 until R5. Is the formulae for calculating r in series and parallel for all 10 lamp combinations acceptable? Explain your answer.
I know the answer is that not all the combinations are accepted but that's all I know