Originally Posted by

**mathlove** thank you so much,grandad and plato.

yes, actually it means that i always need the p fixed things to remain in each of my arrengement,but not necessarily together.

but i have the formula correctly copied.i have checked the language and the formula again...

but i am having a problem with (r permutation p)which means i am finding the no of arrengements of r objects,taken p at a time. it is giving me a strange feeling that i am not fixing the p things in all arrengements.

say i have the letters a,b,c,d,e,f of which i am asked to find the no of 4 letter words i can form ,where each word must have c and e.(ie. n=6,r=4, ,p=2)

now if i only think about the letters other than c and e, i have 2 places to be filled from 4letters -a,b,d,f =4P2

now to put c and e ,i have 3 blank spaces around the 2 words(other than c and e)chosen already by the above process.

now c has 3 options and d also has 3 options, (because they can sit together),so 3*3*4P2?