Given:
An adult takes 800 mg of ibuprofen.
Each hour, one fourth of the medicine leaves the system.
To find:
The exponential equation that models this situation.
Solution:
The general exponential model is
...(i)
where, a is initial value r is decreasing rate and x is time period.
According to the question,
Initial value of medicine = 800 mg
Decreasing at the rate of
per hour.
Putting a=800 and
in (i), we get

Therefore, the required exponential equation that models the given situation is
, where, y is ibuprofen (in mg) after x hours.