About distribution of stochastic variables

Dear all, I have a problem that I can figure out in the following way.

Let me assume to have a distribution (normal or lognormal) of a stochastic variable, and I already know the average and standard deviation of this distribution. Moreover, I know that the sum of X elements of this distribution should be equal to an already fixed and well known number N. How to find X? I am going literally crazy.

Thanks and best regards Matteo