Let's take the actual question section all by itself:
If an object displaces 40 grams of water, then the buoyant force on the object is equal to the weight of 40 grams of water (Archimedes' principle)... which is equal to the weight of 40 grams of ANY substance.
Weight = (mass) x (gravity)
Weight = (0.040 kg) x (9.8 m/s²)
<em>Weight = 0.392 Newton</em>
so that's the buoyant force acting on the object.
Now, to deal with the first phrase of the question ... everything before the first comma ... you said "100 grams is equal to 1 newton of force". This phrase is really quite meaningless, and there's nothing to be done with it.
A quantity of mass is never "equal to" a force.
Fortunately, whatever this is supposed to mean, it's not needed to answer the rest of the question.
