Answer:
The answer I got was 7 7/10
(sorry if wrong!)
You want to find the value of x for which the area under the curve to the left of x is 0.6. One way to do that is to create the cumulative distribution function (CDF) for the given PDF, then see where it is equal to 0.6.
Doing that, we find a = 5.
So P stands for principle which is the amount of the original investment, R stands for rate which is the percent but must be turned into a decimal, T stands for time for example if the time was in a certain amount of months then you plug that in with x/12, and lastly I stands for interest which is the original investment outcome ^_^