23. Exemplar exercise problems XV

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.

 

1. A sunscreen with SPF \(15\) allows only \(\frac{1}{15}\) of the sun’s UV rays. What per cent of UV rays does the sunscreen abort? 92.33 %89.33 %93.33 %91.33 %

 

2. Suppose a sunscreen allows \(25 \%\) of the sun’s UV rays.

 

(a) What fraction of UV rays does this sunscreen block? Give your answer in lowest terms. $\frac{i}{i}$

 

(b) Use your answer from Part (a) to calculate this sunscreen’s SPF. Explain how you found your answer. SPF - 4SPF - 3SPF - 100SPF - 75

 

3. A label on a sunscreen with SPF \(30\) claims that the sunscreen blocks about \(97 \%\) of harmful UV rays. Assuming the SPF factor is accurate, is this claim true? Explain. FalseTrue