Which is a stretch of an exponential decay function?
f(x) =4/5(5/4)^x

f(x) =4/5(4/5 )^x

f(x) =5/4(4/5)^x

f(x) =5/4(5/4)^x

The answer 

for such a question, it is required to graph each function, the answer is 
f(x) =4/5(4/5 )^x 
check the attached file for proof

Let  be the original amount of the substance. After t years, the amount left A is given by:

                               .

For example, after 1 year, the amount is

      .

Thus, after one year, only 0.75=3/4 of the original amount is left.

We want to solve for t, such that   is one-third of , so we set the equation:

.

Simplifying by , we have:

.

This is an exponential equation, so we can solving it by rewriting it as a logarithm:

.

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