To boor everyone with math. SPF is supposedly the inverse of having not sunscreen. Lying in the sun for an hour with SPF 15 is like 1/15 of an hour with no sunscreen and SPF 30 is like 1/30 of an hour with no sunscreen. So and hour at SPF 15 and an hour with SPF 30 is like1/15 + 1/30 = 3/30 = 2/20 or two hours with SPF 20 sunscreen.
To boor everyone with math. SPF is supposedly the inverse of having not sunscreen. Lying in the sun for an hour with SPF 15 is like 1/15 of an hour with no sunscreen and SPF 30 is like 1/30 of an hour with no sunscreen. So and hour at SPF 15 and an hour with SPF 30 is like1/15 + 1/30 = 3/30 = 2/20 or two hours with SPF 20 sunscreen.