13.7 lbs
Since the assumption is that the alloy consists only of nickel and iron, the mass of the nickel will be the total mass of the alloy minus the mass of the iron. So
87.45 kg - 81.25 kg = 6.2 kg
Now we need to convert from kg to lb using the conversion factor of 2.20462 lb/kg. So
6.2 kg * 2.20462 lb/kg = 13.668644 lb.
Rounding to 3 significant figures gives 13.7 lb of nickel was used.
