In this method you can use proportion or formula.

If Profit % is given then selling price will be

CP (100 + P%) SP = ------------------ 100 |

CP (100 - L%) SP = ------------------ 100 |

As profit percent is given, we use the following formula.

CP (100 + P%) SP = ------------------ 100 450 (100 + 5) SP = ------------------ 100 450 x 105 SP = ------------- 100 47250 SP = ----------- 100 |

∴ Selling price = $472.50

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We have, Cost of one copy = $15

∴ Cost of 500 copies = $(500 x 15) = $ 7500

It is given that the bookseller sold 500 copies at a profit of 20%

∴ Profit = 20% of C.P. = 2o/100 x 7500 = $1500

Now, S.P. = C.P. + Profit = 7500 + 1500 = $ 9000

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We have, C.P. of one dozen oranges = $ 5

Loss % = 5%

∴ Loss = 5% of $ 5 = 5/100 x 5= $ ¼

∴ S.P. = C.P. – Loss = 5 – ¼ = $ 19/4

Thus, S.P. of one dozen oranges = $ 19/4

∴ S.P. of one orange = $ 19/4 x 1/12 = $ 0.395 or $40

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