Q112 of 96 Page 287

For an amount, explain why, a 20% increase followed by a 20% decrease is less than the original amount.

Let us assume the amount be RS. 100


⇒ x% increase in amount = amount +


⇒ 20% increase in Rs.100 =


= 100 + 20


= Rs. 120


⇒ x% decrease in amount = amount -


Now, 20% decrease in Rs.120 =


= 120-24


= Rs. 96


Hence, decreased price is less than original amount.


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110

A store is having a 25% discount sale. Sheela has a Rs 50 gift voucher and wants to use it to buy a board game marked for Rs 320. She is not sure how to calculate the concession she will get. The sales clerk has suggested two ways to calculate the amount payable. –

Method 1: Subtract Rs 50 from the price and take 25% off the resulting price. - Method 2: Take 25% off the original price and then subtract Rs 50.


a. Do you think both the methods will give the same result? If not, predict which method will be beneficial for her.


b. For each method, calculate the amount Sheela would have to pay. Show your work.


c. Which method do you think stores actually use? Why?

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Living on your own: Sanjay is looking for one-bedroom apartment on rent. At Neelgiri appartments, rent for the first two months is 20% off. The one bedroom rate at Neelgiri is Rs 6,000 per month. At Savana appartments, the first month is 50% off. The one bedroom rate at Savana appartments is Rs 7000 per month. Which apartment will be cheaper for the first two months? By how much?

 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.

A sunscreen with SPF 15 allows only of the sun’s UV rays. What per cent of UV rays does the sunscreen abort?

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.

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.