Saltpeter is a compound made of potassium(k), nitrogen(n), and oxygen(o). every 100.0g of saltpeter contains 38.67g potassium and 13.86g nitrogen. what is the mass of oxygen contained in 285.0mg of saltpeter? answer needs to be in grams

Use a proportion ... 
100.0g - 38.67g - 13.86g = 47.47g Oxygen 
285.0 mg = 0.285g 
47.47/100 = x/0.285g 
x = ( 47.47/100) X 0.285g 

The  grandmother cell  is a hypothetical  neuronthat represents a complex but specific concept or object.[1]  it activates when a person "sees, hears, or otherwise sensibly discriminates"[2]  a specific entity, such as his or her grandmother. the term was in use at least as early as 1966 amongst staff and students in the department of experimental psychology, university of cambridge, england. a similar concept, that of the  gnostic neuron, was proposed two years later