The five gallons is cheaper since 97.45 divided by 5 is equal to 19.49.
The horizontal asymptote is the limit of the temperature as time goes to infinity.
If we ignore the constant in the denominator, then

And the limit as t-> infinity is 0

This means the temperature will decrease over time approaching 98.6 in the long run.
The answer to this is p=-25/4 and in other ways it’s either p=-6.25, p=-6 1/4 or p=-5/2^2