Statistics: inverse normal on the Casio fx-991ES PLUS C

Tutoring statistics, calculators are important. The tutor mentions how to use inverse normal on the Casio fx-991ES PLUS C.

The following is according to my understanding.

I mention the concept of the inverse normal in my post from January 25, 2026. That post refers to Google Sheets; this post describes how to use it on the Casio fx-991ES PLUS C.

The top-down idea is to press MODE, then down arrow on the circular key at the top, then 3 for the DIST menu, then 3 again for Inverse Normal. From there, the user follows the cues.

Example: imagine the time to wash a car is normally distributed with mean 25 minutes and standard deviation 5 minutes. By what time duration will a wash be completed 45 percent of the time?

Solution: MODE, then down arrow on the circular key at the top, then 3, then 3 again will yield the InvrNorm: Area? prompt. In this case, one would enter 0.45: the area means the proportion of the population less than or equal to the measurement. Moreover, 45 percent is 0.45. Next, InvrNorm: σ? will appear, which asks for the standard deviation: 5 minutes in this case, so the user would enter 5. Next, InvrNorm: μ? appears, which is 25 in this case: the user would enter 25. The answer appears as xInv= 24.37. Therefore, 45 percent of the time, a wash will be completed within 24.37 minutes.

A reminder: to get back from DIST mode to normal mode, it’s MODE 1.

Source:

casio.com: fx-991ES PLUS C User’s Guide