If 20.5 Liters of nitrogen are used, how many liters of hydrogen are also used, and how many liters of ammonia are produced?

P V = n R T

same pressure, same R, same T
so
V is proportional to number of moles, n
N2 + 3H2 ---> 2NH3
so you need 3*20.5 liters of H2
and you get 2*20.5 liters of NH3