Combinatorics problems

[mathdaemon]

Hello everyone,

Please tell me where I am going wrong in the following problems

a) There are m copies each of n different books in a university library.
The number of ways in which >=1 books can be selected is
My Soln :
Total no. of books = n(m+1)
Therefore, 2^n(m+1)-1
(no. of combinations of n different things taken 0 or more at a time is 2ⁿ)
Correct Soln: (m+1)ⁿ-1

b) In how many ways can 5 jobs be assigned to 4 people such that every employee is assigned at least one job.
My Soln :
Every employee 1 job+ one employee 2 jobs
= 5x4x3x2 + <I am not able to figure out this part>
Correct Soln: 240

c) Number of integral solutions of x+y+z=0 with x>=5
y>=-5 and z>=-5
My soln: Infinite because there are many combinations that yield zero
Correct soln: 21

Thanks for your time

P.S. : Sorry if I posted in the wrong place.

Regards