BETAINV function
Returns the inverse of the cumulative beta probability density function for a specified beta distribution. That is, if probability = BETADIST(x,...), then BETAINV(probability,...) = x. The beta distribution can be used in project planning to model probable completion times given an expected completion time and variability.
For more information about the new function, see BETA.INV function.
Syntax
BETAINV(probability,alpha,beta,[A],[B])
The BETAINV function syntax has the following arguments:

Probability Required. A probability associated with the beta distribution.

Alpha Required. A parameter of the distribution.

Beta Required. A parameter the distribution.

A Optional. A lower bound to the interval of x.

B Optional. An upper bound to the interval of x.
Remarks

If any argument is nonnumeric, BETAINV returns the #VALUE! error value.

If alpha ≤ 0 or beta ≤ 0, BETAINV returns the #NUM! error value.

If probability ≤ 0 or probability > 1, BETAINV returns the #NUM! error value.

If you omit values for A and B, BETAINV uses the standard cumulative beta distribution, so that A = 0 and B = 1.
Given a value for probability, BETAINV seeks that value x such that BETADIST(x, alpha, beta, A, B) = probability. Thus, precision of BETAINV depends on precision of BETADIST.
Example
The example may be easier to understand if you copy it to a blank worksheet.
How do I copy an example?

Select the example in this article.
Selecting an example from Help

Press CTRL+C.

In Excel, create a blank workbook or worksheet.

In the worksheet, select cell A1, and press CTRL+V.

To switch between viewing the results and viewing the formulas that return the results, press CTRL+` (grave accent), or on the Formulas tab, in the Formula Auditing group, click the Show Formulas button.
After you copy the example to a blank worksheet, you can adapt it to suit your needs.

