20kg of ammonium nitrate is made from ammonia and nitric acid, what mass of ammonia was used?

The given equation is - NH₃ + HNO₃ --> NH₄NO₃
Equation to use - Moles = Mass/RFM
RFM of ammonium nitrate = 14 + 4 + 14 + (3x16) = 80
20kg = 20000g
Moles of ammonium nitrate = 20000/80 = 250
Mole ratio of ammonium nitrate to ammonia = 1:1
Moles of ammonia = 250
Moles x RFM = mass
RFM of ammonia = 14 + 3 = 17
Mass of ammonia = 17 x 250 = 4250g = 4.25kg