Beauty

[ShakesPeer]

I live in Florida and make a concerted effort to stay out of the sun as much as possible. I'm not well versed in the rules of spf because I stay indoors so much. I currently use philosophy's Supernatural Poreless Flawless spf 15, and philosophy's tinted moisturizer spf 20, not for the spf but for coverage they offer me. I don't like heavy makeup, and these two products work great for my needs. Sometimes I use just one, and sometimes I use both at the same time. It just happens to be convenient that they have spf in them for my trips to and from the car, to the mailbox, etc. I apologize if this is a stupid question, but do the spf's combine at all to increase the strength? When I wear both, am I getting the 20 coverage because it's higher than the 15, or a combination of both? Also, in what order would you apply these if you wore them both at the same time? Last question; it's inevitable that I get stuck outdoors from time to time, so with spf factors this low, how long do I have before I should come in out of the sun? 30 minutes? An hour?

Thank you so much in advance ladies, and I apologize again if I'm asking a really stupid question.