Acertain quantity decays exponentially over time. the initial quantity at t = 0 is 25,000. the quantity decays at a rate of 5%. what is the quantity at t = 3?

In order to sole the problem you need to create an equation to model what you are trying to do.
You end up with an equation looking like
25000*0.95^t
25000 is the starting value when t=0
95% is the amount remaining after the 5% decay
0.95 is being raised to the power of t to determine how many times it has decayed
When t=3 the equation should be
25000*0.95^3=21434.375