This is a relatively straightfoward question, which I view as if it were on a fertilizer box. Random example: The fertilizer box says it contains 15% phosphate (as P2O5), but the actual ingredient contributing the phosphate is tri-ammonium phosphate. It is possible to calculate what weight portion of the tri-ammonium phosphate is P2O5 which would allow me to find the weight percent of the tri-ammonium phosphate in the mix as a whole.
You need to go the other direction. What percent of NH4Cl's mass is from nitrogen? It should be easy from there on...
-UC