Answer:
model:

amount after 6 days:

Explanation:
We can use the exponencial function of growth/decay to model this problem:

Where P(t) is the final value after time t, Po is the inicial value and r is the rate of change.
In our case, the inicial value is 399 grams, the rate is -0.5 (that is, the value decreases by half in each time cycle), and t is the time in days.
So our function will be:

Then, to find the amount of substance after 6 days, we just need to calculate P using t = 6:

