Asample of ammonia gas, nh3, has a volume of 285.0 ml at a pressure of 98.50 kpa. what will the volume of the ammonia be at a pressure of 94.20 kpa? assume that all other properties of the gas remain constant.

We know that an ideal gas keeps the relation between pressure, volume and temperature constant.
    P * V / T = constant
   Then if we have the gas in two diferente state:
   P1*V1/T1 = P2*V2/T2
    Temperature remain constant so then:
   P1*V1 = P2*V2
  98.5*285 = 94.2*V2
  V2 = 98.5*285/94.2 = 298 ml
   So the volume of the ammonia will be 298 ml

Using Avogadro's law 

where: P1V1/n1T1 = P2V2/n2T2 

The problem states it implicitly but n and T are constant 

P1V1 = P2V2 

V2 = P1V1/P2 

P1 = 98.50 kPa 
V1 = 285.0 mL 
P2 = 94.20 kPa 

V2 = (98.50 x 285)/94.20 
V2 = 298.00 mL 

The volume increases to 298 mL.