Question 10458
 1.ASSUMING THAT THE STUDENTS ARE IDENTICAL
 2.ASSUMING THAT THE STUDENTS ARE DISTINGUISHABLE

 1. Like the possible solutions of the integral equations
    x + y + z = 20 (z,y & z as three classrooms)
  Arrange r = 20 possble combinations among n=3 elements.
  There are H(n, r) = C(n+r-1, r) = C(22,20) = C(22,2)
  =22*21/2 = 21*11 = 231

 2. Each student has 3 choices, totally, there 3^20 ways.


 Kenny