## Sunny 16 rule

what math equation do you use to determine the number of stops darker in doors than outdoors if a photograph is properly exposed photograph is taken indoors at ISO 800, f/2.8, and at 1/50 of a second ?
Im trying to calculate sunny 16 rule and compare how much darker it is.
For instance if I do Sunny 16 rule on one picture
and use f2.8 ISO 640 and 1/50 second, is this how to calculate this?
f/16 is 5 times darker. And for reference I pick out 100, 1/1000.
5 times lighter for iso. and 4 times lighter for shutter. So it would be 4 times lighter?
I don't see 1/50 second for shutter speed though.
I have seen these 2 questions posted on another forum. It appeared to me that the question is a homework question rather something the poster really wanted to know. It it is the case how would the instructor taught the students to get answer to these kind of question. I know there are many ways to come up with the right answer but someone who teaches a photography class and put such a question in the homework how would that person teaches the students to solve the problem?