Hello,

If you like to put together your probability distribution
stepwise, you can use my user-defined functions redw() or
rww() at
http://www.sulprobil.com/html/random_numbers.html, for
example.

If, for example, X=10, Y=20 and your desired Z=13, then
you could (there are plenty of distribution which result
in a mean of 13) take

=redw(9,1)*10+10

because it will (on average!) result in (9*(10+15)/2+1*
(15+20)/2)/10 = 13.

HTH,
Bernd