Question

Sunscreens block harmful ultraviolet (UV) rays produced by the sun. Each sunscreen has a Sun Protection Factor (SPF) that tells you how many minutes you can stay in the sun before you receive one minute of burning UV rays. For example, if you apply sunscreen with SPF 15, you get 1 minute of UV rays for every 15 minutes you stay in the sun.

Suppose a sunscreen allows 25% of the sun’s UV rays.

1. What fraction of UV rays does this sunscreen block? Give your answer in lowest terms.
2. Use your answer from Part (a) to calculate this sunscreen’s SPF. Explain how you found your answer.

Answer 1

a. Sunscreen allows 25% of the sun’s UV rays.

∴ It blocks UV rays = 100 – 25 = 75%

= `75/100`

= `3/4`

b. Sunscreen allows 25% on `3/4` of UV rays.

It means, it protect = `1 - 3/4 = 1/4` of UV rays.

Hence, it’s a SPF 4.