In maths terms, SPF 25 would offer less protection than 15. Why? Well, 25 has three factors (1, 5, 25) whereas 15 has four factors (1, 3, 5, 15).
Joking aside, there is some serious maths here.
The SPF is calculated by comparing the amount of time needed to produce sunburn on protected skin to the amount of time on unprotected skin. I'm fair-haired and turn red after about 10 minutes in the sun. This is my initial burning time.
If I use a bottle of SPF 2 then it will take 20 minutes for my skin to turn red. If I use a lotion with a factor of 30 then it multiplies my initial burning time of 10 minutes by 30. This means I'll change colour after 18,000 seconds, 300 minutes or five hours, depending on your preference.
Think of the numbers and calculations you could squeeze out of a bottle of sun lotion over the summer term. Hip, hip, hooray
John Dabell is a numeracy consultant and teacher trainer