if the pressure of n2/h2 in aclosed apparatus is 100 atm and 20 percent of the mixture then react ,the pressure at the same temperature would be
Is the initial mixture of N2 and H2 stoichiometric (1:3) ratio? That would make a difference in the answer.
There are other possible N-H compounds besides NH3.
This does not appear to be a well posed problem, but I am not a chemistry expert.