Answer:
a) 
And for this case we can rewrite the model like this:

If we integrate both sides we got:

If we use exponentials for both sides we got:

For this case
and r = -0.16
So then our model would be given by:

Where t represent the number of hours
b) 
Step-by-step explanation:
Part a
For this case we can assume the proportional model given by:

And for this case we can rewrite the model like this:

If we integrate both sides we got:

If we use exponentials for both sides we got:

For this case
and r = -0.16
So then our model would be given by:

Where t represent the number of hours
Part b
For this case we can replace the value t=5 into the model and we got:
