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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 1/15 of the sun’s UV rays. What percent of UV rays does the sunscreen abort
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
b. Use your answer from Part (a) to calculate this sunscreen’s SPF. Explain how you found your answer
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

Solution:

1. The sunscreen aborts 14/15 of the sun rays. In percentage terms the Sunscreen with SPF of 15 aborts 93.33 percent of the UV rays.

2. (a) If the sunscreen allows 25% of the Sun’s UV rays, that implies that 75% of the time the sunscreen blocks or aborts the UV rays. In other words 3/4th of the UV rays are blocked by the sunscreen. (b) The Sunscreen SPF is 4 which implies that one can stay in the sun for four minutes before one gets exposed to one minute of UV rays.

3. SPF 30 implies that a person can stay exposed to the Sun for 30 minutes before he or she gets exposed to one minute of UV rays which in percentage terms means the sunscreen allows only (1/30) × 100 = 3 1/3 % of the UV rays and blocks approximately 97% of the harmful UV rays. The claim is true

✦ Try This: 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 10 allows only 1/10 of the sun’s UV rays. What percent of UV rays does the sunscreen abort?

2. Suppose a sunscreen allows 20% of the sun’s UV rays.

3. What fraction of UV rays does this sunscreen block? Give your answer in lowest terms.

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

5. A label on a sunscreen with SPF 50 claims that the sunscreen blocks exactly 98% of harmful UV rays. Assuming the SPF factor is accurate, Is this claim true? Explain.

1. The sunscreen aborts 9/10 of the sun rays. In percentage terms the Sunscreen with SPF of 10 aborts 90 percent of the UV rays.

2. (a) If the sunscreen allows 20% of the Sun’s UV rays, that implies that 80% of the time the sunscreen blocks or aborts the UV rays. In other words 4/5th of the UV rays are blocked by the sunscreen.(b) The Sunscreen SPF is 5 which implies that one can stay in the sun for five minutes before one gets exposed to one minute of UV rays.

3. SPF 50 implies that a person can stay exposed to the Sun for 30 minutes before he or she gets exposed to one minute of UV rays which in percentage terms means the sunscreen allows only (1/50) × 100 = 2 % of the UV rays and blocks exactly 98% of the harmful UV rays. The claim is true.

☛ Also Check: NCERT Solutions for Class 8 Maths

NCERT Exemplar Class 8 Maths Chapter 9 Problem 113

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 1/15 of the sun’s UV rays. What percent of UV rays does the sunscreen abort, 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, b. Use your answer from Part (a) to calculate this sunscreen’s SPF. Explain how you found your answer, 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

Summary:

A sunscreen with SPF 15 allows only 1/15 of the sun’s UV rays. 93.33% percent of UV rays does the sunscreen aborts, 2. If a sunscreen allows 25% of the sun’s UV rays, a. It implies 3/4 of UV rays are blocked by the sunscreen, b. The sunscreen’s SPF is 4 which implies that a person has to stay out in the sun for 4 minutes before he or she gets exposed to one minute of UV rays, 3. A label on a sunscreen with SPF 30 claims that the sunscreen , blocks about 97% of harmful UV rays which means the claim is true

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