Answer:

Explanation:
As the initial amount decreases by half every 3.5 hours, considering
as the initial amount you have that:
at t=0 the amount would be

at t=3.5 the amount would be

at t=7 the amount would be

at t=10.5 the amount would be

and so on...
this behavior can the modeled as:

This matches all the points described above.