Generate random percentages and placing vectors

Hello everyone,

I have two questions:

1.) I would like to generate random percentages that add up to 100.  
For
example, if I need 5 percentages, I would obtain something like: 20,  
30, 40,
5, 5. Is there some way to do this in R?

2.) I would like to insert vectors of specified length into a larger  
vector
of specified length randomly, and fill the gaps with zeroes. For  
example, if
I have 3 vectors of length 3, 2, and 2 with values and I would like to
randomly place them into a vector of length 25 made of 0's.

Thank you in advance!